.github/workflows/ci.yml

@@ -62,7 +62,7 @@ jobs:
             libxcb-image0 libxcb-keysyms1 libxcb-randr0 libxcb-render-util0 \
             libxcb-shape0 libxcb-sync1 libxcb-xfixes0 libxcb-xinerama0 \
             libxcb-cursor0 libdbus-1-3 libgl1 libxkbcommon0 xvfb
-      - run: pip install -e ".[dev]"
+      - run: pip install -e ".[gui,dev]"
       - run: xvfb-run -a pytest tests/gui -m "not slow"
 
   test-integration:

.gitignore

@@ -61,3 +61,7 @@ examples/ozan*.osmodel
 examples/ozan_results/
 /Ex*.csv
 /Ex*.png
+
+# BEGIN oh-my-opencode-slim clonedeps
+.slim/clonedeps/repos/
+# END oh-my-opencode-slim clonedeps

.ignore

@@ -0,0 +1,9 @@
+# BEGIN oh-my-opencode-slim clonedeps
+!.slim/
+!.slim/clonedeps.json
+!.slim/clonedeps/
+!.slim/clonedeps/repos/
+!.slim/clonedeps/repos/**
+.slim/clonedeps/repos/**/.git/
+.slim/clonedeps/repos/**/.git/**
+# END oh-my-opencode-slim clonedeps

.slim/clonedeps.json

@@ -0,0 +1,15 @@
+{
+  "version": "1.0.0",
+  "updatedAt": "2026-09-16T17:30:00.000Z",
+  "dependencies": [
+    {
+      "name": "opstool",
+      "resolvedVersion": "1.0.26",
+      "repoUrl": "https://github.com/yexiang92/opstool.git",
+      "ref": "v1.0.26",
+      "path": ".slim/clonedeps/repos/yexiang92__opstool",
+      "packagePath": "opstool/vis",
+      "reason": "Read-only source for evaluating whether to port opstool's PyVista/Plotly visualization settings into otko's canvas backends, or to depend on opstool.vis directly (GPL-3.0, compatible with otko's AGPL-3.0 via GPLv3 section 13)."
+    }
+  ]
+}

AGENTS.md

@@ -5,7 +5,7 @@ Pre-alpha SAP2000-style desktop GUI for OpenSeesPy. Python 3.10+; **Windows requ
 ## Install
 
 ```bash
-pip install -e ".[gui,dev]"  # desktop: Qt + PyVista + dev tools
+pip install -e ".[gui,dev]"  # desktop: Qt + PyVista + plotly.js backend + dev tools
 pip install -e .              # headless: core + services only, no Qt (scripts, notebooks, web backends)
 python -m otko                # launch GUI (src/otko/app.py:run)
 ```
@@ -19,6 +19,8 @@ Strict one-way MVVM + services: `views → viewmodels → services → core`.
 - `core/` (entities: `project.py`, `geometry/`, `materials/`, `sections/`, `loads/`, `analysis/`, `catalog/`): stdlib + numpy + pydantic only. **No Qt, no openseespy. Period.**
 - `services/` (`opensees_runner.py`, `persistence.py`, `results.py`, ...): may use core + h5py + openseespy. **No Qt.**
 - `views/`: PySide6/pyvistaqt only. **No direct `import openseespy`** — go through a service.
+- `views/canvas3d/` (**PyVista/VTK**, default) and `views/canvas_plotly/` (plotly.js in a `QWebEngineView`) are two backends for the same central 3D view. Both satisfy the `CanvasBackend` protocol in `views/canvas_base.py`, share one `SelectionState` owned by `MainWindow`, and are swapped live via **Options → Canvas Backend** (persisted in `QSettings` under `canvas/backend`). `MainWindow._activate_canvas` keeps both widgets in a `QStackedWidget` — never destroy a canvas mid-session (VTK leaves dangling make-current callbacks). Backend-specific gaps are declared by `CanvasCapabilities` (e.g. Plotly has no force-diagram overlay or video export yet); gate UI on `canvas.capabilities`, never on the backend name. `canvas_plotly/trace_builder.py` is pure (no Qt, no pyvista) and unit-tested headless. `PlotlyCanvas` pushes with `Plotly.react`, which resets scene attributes the layout omits: data-only pushes pass `preserveView` so `html.py` carries the live camera and padded axis ranges forward, and the hover/snap marker is restyled only when its visible state changes (a restyle per hover event redraws per mouse move until the stack blows). Regression tests: `tests/gui/test_plotly_view_preservation.py`, `tests/gui/test_plotly_hover.py`.
+- `views/canvas3d/style.py` (`RenderStyle`) is the single source of truth for colours/sizes on **both** backends; renderers must read it rather than hard-coding a colour. The `STYLE_FIELDS` subset is user-editable via **Options → Plot Properties…** (`views/dialogs/plot_properties.py`), previews live through `MainWindow.set_plot_style(..., persist=False)`, and persists as JSON in `QSettings` under `plot/props`.
 - `viewmodels/` bridges core↔Qt (signals, `QUndoStack`); `commands/` holds `QUndoCommand` subclasses.
 - Rules: public functions need type hints + docstring; new domain entities go through Pydantic validation; ops >50 ms run off the GUI thread (`AnalysisWorker` in QThread, cancel via `isInterruptionRequested()`, results cross threads as lightweight `ResultsHandle` to HDF5).
 
@@ -56,3 +58,17 @@ git remote add origin ssh://git@smill-home.ddns.net/smill/otko.git
 - `examples/*.py` are source of truth; `examples/*.osmodel` are generated artifacts (checked in). Never hand-edit `.osmodel` — change the script and regen: `python examples/cantilever.py` (each script saves, reloads, asserts clean round-trip).
 - Projects persist as single Pydantic-validated JSON `.osmodel` (diffable); analysis output goes to `*.osresults.h5` (HDF5, one group per case).
 - Quick smoke: open `examples/cantilever.osmodel` → run `Tip-Load` → M3 peaks 50 kN·m at fixed end.
+
+## Cloned Dependency Source
+
+Read-only dependency source repositories are available under
+`.slim/clonedeps/repos/` for inspection. Do not edit these clones. The
+structured manifest is `.slim/clonedeps.json`.
+
+- `.slim/clonedeps/repos/yexiang92__opstool/` — `yexiang92/opstool` at `v1.0.26`; the OpenSeesPy
+  pre/post-processor whose PyVista and Plotly visualization settings
+  (`opstool/vis/{pyvista,plotly}/plot_utils.py`, `plot_resp_base.py`, `vis_model.py`) are the
+  reference for otko's canvas look-and-feel. GPL-3.0, and GPLv3 §13 explicitly permits combining
+  it with otko's AGPL-3.0. Any code actually ported from it must keep the opstool copyright notice
+  and record that it was modified (GPLv3 §5a/b) — add that entry to `NOTICE` when the port lands.
+

NOTICE

@@ -45,6 +45,21 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 ```
 
+## Ported / adapted code: `opstool`
+
+The canvas element colour palette in `src/otko/views/canvas3d/style.py`
+(per-family element colours and the diverging response colour scale) and the
+resulting diagram colouring in `views/canvas3d/diagram_renderer.py` were
+adapted from the `opstool` project, which is distributed under the **GNU
+General Public License v3.0**. opstool is Copyright © Yexiang Yan and
+contributors.
+
+In accordance with GPLv3 §5(a)/(b) this notice records that the material was
+modified and adapted for OTKO. Combining the GPLv3-covered material with
+OTKO's AGPL-3.0 code is permitted by GPLv3 §13; the combined work is
+conveyed under AGPL-3.0, and the GPLv3 terms continue to apply to the
+opstool-derived portions.
+
 ## Runtime dependencies
 
 OTKO depends on third-party software that is not covered by OTKO's

pyproject.toml

@@ -42,6 +42,7 @@ gui = [
     "vtk>=9.3",
     "pyqtgraph>=0.13",
     "imageio[ffmpeg]>=2.34",
+    "plotly>=5.18",
 ]
 dev = [
     "pytest>=7.4",

src/otko/views/canvas3d/diagram_renderer.py

@@ -31,6 +31,7 @@ import pyvista as pv
 
 from otko.core import Project
 from otko.services.element_forces import DiagramData, ForceComponent
+from otko.views.canvas3d.style import RenderStyle
 
 _LOG = logging.getLogger("otko.diagram")
 
@@ -62,8 +63,9 @@ class DiagramRenderer:
     - ``clear()`` removes the overlay.
     """
 
-    def __init__(self, plotter: Any) -> None:
+    def __init__(self, plotter: Any, style: RenderStyle | None = None) -> None:
         self._plotter = plotter
+        self._style = style or RenderStyle()
         self._actor: Any = None
         self._label_actor: Any = None
 
@@ -158,7 +160,9 @@ class DiagramRenderer:
         self._actor = self._plotter.add_mesh(
             mesh,
             scalars="value",
-            cmap="coolwarm",
+            # Shared response palette (opstool's default diverging scale), so
+            # the PyVista and Plotly backends stay visually consistent.
+            cmap=list(self._style.response_scale_colors),
             clim=(-vmax, vmax),
             show_scalar_bar=True,
             scalar_bar_args={"title": comp_label, "n_labels": 5},

src/otko/views/canvas3d/model_canvas.py

@@ -24,6 +24,7 @@ from otko.core import Project
 from otko.views.canvas3d.model_renderer import ModelRenderer
 from otko.views.canvas3d.selection import SelectionState
 from otko.views.canvas3d.style import RenderStyle
+from otko.views.canvas_base import CanvasCapabilities
 
 PICK_DEBUG = False
 
@@ -31,6 +32,9 @@ PICK_DEBUG = False
 class ModelCanvas(QtInteractor):  # type: ignore[misc]
     """The central 3D viewport widget."""
 
+    #: VTK supports every optional overlay layer.
+    capabilities = CanvasCapabilities()
+
     # Convenience signals re-emitted from SelectionState.
     nodePicked = Signal(int)
     elementPicked = Signal(int)
@@ -338,6 +342,17 @@ class ModelCanvas(QtInteractor):  # type: ignore[misc]
         self._renderer.set_show_local_axes(enabled)
         self.render()
 
+    def set_style(self, style: RenderStyle) -> None:
+        """Swap the visual style and repaint the scene.
+
+        Applies the background immediately (it lives outside the renderer)
+        and rebuilds the actors whose colours come from the style LUTs.
+        """
+        self._style = style
+        self.set_background(style.background_bottom, top=style.background_top)
+        self._renderer.set_style(style)
+        self.render()
+
     def set_display_options(self, *, show_node_labels: bool, show_element_labels: bool) -> None:
         """Toggle viewport labels such as node and element names."""
         self._renderer.set_display_options(

src/otko/views/canvas3d/model_renderer.py

@@ -13,7 +13,7 @@ from __future__ import annotations
 import contextlib
 import enum
 from dataclasses import dataclass
-from typing import Any, ClassVar
+from typing import Any
 
 import numpy as np
 import pyvista as pv
@@ -33,7 +33,12 @@ from otko.core import (
     ZeroLengthElement,
     ZeroLengthSectionElement,
 )
-from otko.views.canvas3d.style import RenderStyle
+from otko.views.canvas3d.style import (
+    SELECTED_STATE,
+    RenderStyle,
+    element_family_index,
+    family_palette,
+)
 
 
 class RendererMode(enum.Enum):
@@ -185,9 +190,6 @@ class ModelRenderer:
         r, g, b = (int(round(x * 255)) for x in rgb)
         return f"#{r:02x}{g:02x}{b:02x}"
 
-    _NODE_LUT: ClassVar[list[str]] = ["#d9d9d9", "#00ffff"]  # gray normal, cyan selected
-    _FRAME_LUT: ClassVar[list[str]] = ["#338cd9", "#00ffff"]  # blue normal, cyan selected
-
     def __init__(self, plotter: Any, style: RenderStyle | None = None) -> None:
         self._plotter = plotter
         self._style = style or RenderStyle()
@@ -206,6 +208,8 @@ class ModelRenderer:
         self._frame_actor: Any = None
         self._frame_ids_ordered: list[int] = []
         self._frame_id_to_row: dict[int, int] = {}
+        #: Per-frame palette index (family), parallel to ``_frame_ids_ordered``.
+        self._frame_family: list[int] = []
         self._node_label_actor: Any = None
         self._element_label_actor: Any = None
         self._show_node_labels: bool = False
@@ -278,6 +282,16 @@ class ModelRenderer:
         if self._project is not None:
             self.render(self._project)
 
+    def set_style(self, style: RenderStyle) -> None:
+        """Swap the visual style and rebuild the scene it colours.
+
+        Colours are baked into actor LUTs at build time, so a style change
+        needs a full re-render rather than a scalar-array tweak.
+        """
+        self._style = style
+        if self._project is not None:
+            self.render(self._project)
+
     def set_working_plane(self, plane: tuple[str, float] | None) -> None:
         """Update the working-plane filter and rebuild the grid overlay.
 
@@ -306,11 +320,12 @@ class ModelRenderer:
             self._reglyph_nodes()
 
         if self._frame_pd is not None and self._frame_ids_ordered:
-            states = np.zeros(len(self._frame_ids_ordered), dtype=np.int8)
+            # Restore each cell to its family slot, then flag the selected ones.
+            states = np.array(self._frame_family, dtype=np.int8)
             for eid in element_ids:
                 row = self._frame_id_to_row.get(eid)
                 if row is not None:
-                    states[row] = 1
+                    states[row] = SELECTED_STATE
             self._frame_pd.cell_data["_oss_state"] = states
             self._frame_pd.Modified()
 
@@ -379,7 +394,7 @@ class ModelRenderer:
         self._node_actor = self._plotter.add_mesh(
             glyph,
             scalars="_oss_state",
-            cmap=self._NODE_LUT,
+            cmap=[self._style.node_color, self._style.node_selected_color],
             clim=[0, 1],
             show_scalar_bar=False,
             pickable=True,
@@ -392,6 +407,7 @@ class ModelRenderer:
             return
         cells: list[int] = []
         ids: list[int] = []
+        families: list[int] = []
         for el in frames:
             try:
                 i = self._node_id_to_row[el.nodes[0]]
@@ -400,6 +416,7 @@ class ModelRenderer:
                 continue
             cells.extend([2, i, j])
             ids.append(el.id)
+            families.append(element_family_index(el))
         if not ids:
             return
         pd = pv.PolyData()
@@ -407,16 +424,18 @@ class ModelRenderer:
         pd.lines = np.array(cells, dtype=np.int64)
         pd.cell_data["_oss_id"] = np.array(ids, dtype=np.int64)
         pd.cell_data["_oss_kind"] = np.array(["element"] * len(ids), dtype=object)
-        pd.cell_data["_oss_state"] = np.zeros(len(ids), dtype=np.int8)
+        # Cell scalar carries the palette slot (family, or SELECTED_STATE).
+        pd.cell_data["_oss_state"] = np.array(families, dtype=np.int8)
 
         self._frame_pd = pd
         self._frame_ids_ordered = ids
         self._frame_id_to_row = {eid: i for i, eid in enumerate(ids)}
+        self._frame_family = families
         self._frame_actor = self._plotter.add_mesh(
             pd,
             scalars="_oss_state",
-            cmap=self._FRAME_LUT,
-            clim=[0, 1],
+            cmap=family_palette(self._style),
+            clim=[0, SELECTED_STATE],
             show_scalar_bar=False,
             line_width=3.0,
             pickable=True,
@@ -761,8 +780,8 @@ class ModelRenderer:
         """Add one semi-transparent steel-blue non-pickable aux actor."""
         actor = self._plotter.add_mesh(
             mesh,
-            color=(0.35, 0.60, 0.85),  # cool steel-blue
-            opacity=0.22,
+            color=self._style.extrusion_color,
+            opacity=self._style.extrusion_opacity,
             show_edges=True,
             edge_color=(0.15, 0.25, 0.45),
             line_width=1.0,
@@ -982,7 +1001,7 @@ class ModelRenderer:
         ndf = project.ndf
         dof_idx = _dof_indices(ndf)
         size = max(self._diag_of_points(self._node_original_points) * 0.015, 1e-6)
-        support_color = (1.0, 0.5, 0.1)
+        support_color = self._style.support_color
         for node in project.nodes:
             if not any(node.restraint[i] for i in dof_idx):
                 continue
@@ -996,7 +1015,7 @@ class ModelRenderer:
         if not project.load_patterns or self._node_original_points is None:
             return
         scale = max(self._diag_of_points(self._node_original_points) * 0.05, 1e-6)
-        load_color = (0.2, 0.85, 0.2)
+        load_color = self._style.nodal_load_color
         node_by_id = {n.id: n for n in project.nodes}
         elem_by_id = {e.id: e for e in project.elements}
         for pattern in project.load_patterns:
@@ -1027,7 +1046,7 @@ class ModelRenderer:
             # Draw N arrows along the element span, each perpendicular
             # to the axis in the direction of the load. Uses the same
             # orange-green palette as nodal loads but with shorter arrows.
-            elem_load_color = (1.0, 0.55, 0.2)  # orange
+            elem_load_color = self._style.element_load_color  # distributed loads
             n_arrows_per_elem = 5
             for eload in pattern.element_loads:
                 if not isinstance(eload, UniformElementLoad):
@@ -1116,6 +1135,7 @@ class ModelRenderer:
         self._node_ids_ordered = []
         self._node_id_to_row = {}
         self._frame_ids_ordered = []
+        self._frame_family = []
         self._frame_id_to_row = {}
         self._node_original_points = None
         self._deformation = None
@@ -1148,7 +1168,7 @@ class ModelRenderer:
         return self._plotter.add_point_labels(
             pts,
             labels,
-            font_size=12,
+            font_size=self._style.label_font_size,
             shape_opacity=0.15,
             text_color="black",
             point_color="white",
@@ -1178,7 +1198,7 @@ class ModelRenderer:
         return self._plotter.add_point_labels(
             np.asarray(centers),
             labels,
-            font_size=12,
+            font_size=self._style.label_font_size,
             shape_opacity=0.15,
             text_color="black",
             point_color="white",

src/otko/views/canvas3d/style.py

@@ -4,11 +4,64 @@ All colors, sizes, and glyph parameters live here. Changes apply
 globally through the renderer; downstream code never hard-codes a
 color or radius. A future "theme" feature can simply swap a different
 RenderStyle instance.
+
+The subset in :attr:`RenderStyle.EDITABLE` is surfaced to the user through
+Options → Plot Properties… and persisted in ``QSettings`` (``plot/props``).
+Palette values marked "opstool" are adapted from opstool v1.0.26 (GPL-3.0);
+see ``NOTICE``.
 """
 
 from __future__ import annotations
 
-from dataclasses import dataclass
+from dataclasses import dataclass, replace
+from typing import Any, ClassVar
+
+from otko.core import CorotTrussElement, TrussElement, ZeroLengthElement
+
+#: Selected elements / nodes occupy the last slot of the palette LUT.
+SELECTED_STATE: int = 3
+
+
+def element_family_index(element: object) -> int:
+    """Palette slot for a frame element: 0 beam, 1 truss, 2 link/zero-length."""
+    if isinstance(element, TrussElement | CorotTrussElement):
+        return 1
+    if isinstance(element, ZeroLengthElement):
+        return 2
+    return 0
+
+
+def family_palette(style: RenderStyle) -> list[str]:
+    """LUT shared by both backends: [beam, truss, link, selected]."""
+    return [
+        style.element_beam_color,
+        style.element_truss_color,
+        style.element_link_color,
+        style.selected_color,
+    ]
+
+
+#: Colours / sizes the Plot Properties dialog exposes, in display order.
+STYLE_FIELDS: tuple[tuple[str, str, str], ...] = (
+    ("element_beam_color", "Beam elements", "color"),
+    ("element_truss_color", "Truss elements", "color"),
+    ("element_link_color", "Link / zero-length", "color"),
+    ("selected_color", "Selection highlight", "color"),
+    ("node_color", "Nodes", "color"),
+    ("node_selected_color", "Selected nodes", "color"),
+    ("support_color", "Supports", "color"),
+    ("nodal_load_color", "Nodal loads", "color"),
+    ("element_load_color", "Element loads", "color"),
+    ("extrusion_color", "Section extrusions", "color"),
+    ("extrusion_opacity", "Extrusion opacity", "float"),
+    ("background_top", "Background (top)", "color"),
+    ("background_bottom", "Background (bottom)", "color"),
+    ("show_axis_outline", "Axis outline (grid + ticks)", "bool"),
+    ("label_font_size", "Label font size", "int"),
+)
+
+#: Field names the dialog edits and that are persisted between sessions.
+EDITABLE_FIELDS: tuple[str, ...] = tuple(name for name, _label, _kind in STYLE_FIELDS)
 
 
 @dataclass(frozen=True, slots=True)
@@ -19,20 +72,58 @@ class RenderStyle:
     background_top: str = "#dbe2ef"
     background_bottom: str = "#f5f7fb"
 
-    node_color: str = "#f0a500"  # warm gold
-    node_selected_color: str = "#00d4ff"  # bright cyan
+    node_color: str = "#d9d9d9"  # normal node sphere / marker
+    node_selected_color: str = "#00ffff"  # selected node
 
     frame_color: str = "#1f1f1f"
-    truss_color: str = "#2e5cb8"
+    truss_color: str = "#2e5cb8"  # also the Z-axis colour of the view triad
     zerolength_color: str = "#a020f0"
-    selected_color: str = "#00d4ff"
+    selected_color: str = "#00ffff"  # selected element highlight
+
+    # ── element palette (adapted from opstool v1.0.26, GPL-3.0) ───────
+    # opstool colours wireframe elements by *family* rather than painting
+    # every frame the same colour, which makes mixed beam/truss/link models
+    # readable at a glance. Values come from ``PLOT_ARGS_DEFAULT`` in
+    # ``opstool/vis/{pyvista,plotly}/plot_utils.py``; see ``NOTICE``.
+    element_beam_color: str = "#0652ff"  # beam-column family
+    element_truss_color: str = "#FF8C00"  # truss family
+    element_link_color: str = "#39FF14"  # zero-length / link family
+
+    # ── overlay colours ─────────────────────────────────────────────
+    support_color: str = "#ff8019"
+    nodal_load_color: str = "#33d933"
+    element_load_color: str = "#ff8c33"
+    extrusion_color: str = "#598cff"
+    extrusion_opacity: float = 0.22
+    label_font_size: int = 12
+
+    #: Draw the plotly scene grid + tick marks around the model (the "outline"
+    #: of opstool's scene recipe). Off by default to keep the SAP2000-like
+    #: clean viewport; the coloured X/Y/Z axis lines always stay visible.
+    show_axis_outline: bool = False
+
+    #: Diverging scale for scalar response overlays (force diagrams today,
+    #: nodal / element response plots later): blue → red, evenly spaced.
+    #: This is opstool's ``default_cmap`` (RdYlBu reversed).
+    response_scale_colors: tuple[str, ...] = (
+        "#313695",
+        "#4575b4",
+        "#74add1",
+        "#abd9e9",
+        "#e0f3f8",
+        "#fee090",
+        "#fdae61",
+        "#f46d43",
+        "#d73027",
+        "#a50026",
+    )
 
     fix_color: str = "#c0392b"  # firebrick
     pin_color: str = "#c0392b"
     roller_color: str = "#e67e22"
     custom_support_color: str = "#7f8c8d"
 
-    load_color: str = "#27ae60"
+    load_color: str = "#27ae60"  # Y-axis colour of the view triad
     mass_color: str = "#9b59b6"
 
     # ── sizes (relative to bbox diagonal unless absolute) ────────────
@@ -49,3 +140,24 @@ class RenderStyle:
     load_min_length: float = 0.5
 
     selection_thickness_factor: float = 1.6  # multiplier for selected actors
+
+    #: Names of the fields the Plot Properties dialog edits.
+    EDITABLE: ClassVar[tuple[str, ...]] = EDITABLE_FIELDS
+
+    # ── helpers ──────────────────────────────────────────────────────
+    def response_colorscale(self) -> list[tuple[float, str]]:
+        """Plotly ``colorscale`` form of :attr:`response_scale_colors`."""
+        count = len(self.response_scale_colors)
+        if count < 2:
+            return [(0.0, self.response_scale_colors[0])]
+        return [
+            (index / (count - 1), color) for index, color in enumerate(self.response_scale_colors)
+        ]
+
+    def with_updates(self, **changes: Any) -> RenderStyle:
+        """Return a copy with ``changes`` applied (frozen-dataclass safe)."""
+        return replace(self, **changes)
+
+    def editable_values(self) -> dict[str, Any]:
+        """The dialog-managed subset of this style, as a plain dict."""
+        return {name: getattr(self, name) for name in self.EDITABLE}

src/otko/views/canvas_base.py

@@ -0,0 +1,92 @@
+"""Backend-agnostic contract for the central 3D canvas.
+
+Two backends implement it:
+
+- :class:`otko.views.canvas3d.ModelCanvas` — PyVista/VTK (native OpenGL, default)
+- :class:`otko.views.canvas_plotly.PlotlyCanvas` — plotly.js in a QWebEngineView
+
+The ``MainWindow`` swaps between them at runtime, so both must expose the same
+public surface (signals, selection, working plane, view presets, display
+toggles) and declare what optional overlay layers they support via
+:class:`CanvasCapabilities`.
+"""
+
+from __future__ import annotations
+
+import importlib.util
+from dataclasses import dataclass
+from typing import Any, Protocol
+
+
+def plotly_available() -> bool:
+    """True when the optional ``plotly`` package can be imported.
+
+    The Plotly backend needs plotly.js (shipped inside the ``plotly`` wheel)
+    to render; without it the backend menu entry is disabled.
+    """
+    return importlib.util.find_spec("plotly") is not None
+
+
+@dataclass(frozen=True)
+class CanvasCapabilities:
+    """Optional overlay layers a backend may or may not implement.
+
+    Consumers query these (see ``RenderControls._refresh_action_enablement``)
+    instead of assuming a feature works on every backend.
+    """
+
+    #: Force-diagram ribbon overlay (N/V/M diagrams).
+    diagrams: bool = True
+    #: SAP2000-style section extrusion overlay.
+    extrusions: bool = True
+    #: Per-element local-axis triad overlay.
+    local_axes: bool = True
+    #: Node / element text labels.
+    labels: bool = True
+    #: Off-screen frame capture (mode-shape / time-history video export).
+    animation_export: bool = True
+
+
+class CanvasBackend(Protocol):
+    """Structural contract implemented by every canvas backend.
+
+    This mirrors :class:`ModelCanvas`; it exists so the swap path and the
+    consumers have one written-down interface to code against.
+    """
+
+    capabilities: CanvasCapabilities
+    selection: Any
+
+    def show_project(self, project: Any) -> None: ...
+
+    def clear_model(self) -> None: ...
+
+    def render(self) -> None: ...
+
+    def reset_camera(self) -> None: ...
+
+    def view_isometric(self) -> None: ...
+
+    def view_xy(self) -> None: ...
+
+    def view_xz(self) -> None: ...
+
+    def view_yz(self) -> None: ...
+
+    def set_working_plane(self, plane: str, offset: float) -> None: ...
+
+    def clear_working_plane(self) -> None: ...
+
+    def working_plane_type(self) -> str | None: ...
+
+    def working_plane_offset(self) -> float | None: ...
+
+    def set_snap_preview_enabled(self, enabled: bool) -> None: ...
+
+    def set_show_section_extrusions(self, enabled: bool) -> None: ...
+
+    def set_show_local_axes(self, enabled: bool) -> None: ...
+
+    def set_display_options(self, *, show_node_labels: bool, show_element_labels: bool) -> None: ...
+
+    def set_default_selection_enabled(self, enabled: bool) -> None: ...

src/otko/views/canvas_plotly/__init__.py

@@ -0,0 +1,26 @@
+"""Plotly-backed canvas package (optional GUI backend).
+
+``PlotlyCanvas`` is exported lazily (PEP 562): the pure
+:mod:`~otko.views.canvas_plotly.trace_builder` must stay importable in the
+headless test job, which has no Qt at all.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from otko.views.canvas_plotly.trace_builder import (
+    PlotlyTraceBuilder,
+    Scene,
+    SceneOptions,
+)
+
+__all__ = ["PlotlyCanvas", "PlotlyTraceBuilder", "Scene", "SceneOptions"]
+
+
+def __getattr__(name: str) -> Any:
+    if name == "PlotlyCanvas":
+        from otko.views.canvas_plotly.plotly_canvas import PlotlyCanvas
+
+        return PlotlyCanvas
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

src/otko/views/canvas_plotly/bridge.py

@@ -0,0 +1,28 @@
+"""Qt ↔ plotly.js bridge exposed through ``QWebChannel``.
+
+The page calls :meth:`_Bridge.picked` / :meth:`_Bridge.snapClicked`; every
+call is re-emitted here and wired to the canvas' own Qt signals.
+"""
+
+from __future__ import annotations
+
+from PySide6.QtCore import QObject, Signal, Slot
+
+
+class _Bridge(QObject):
+    """Slot surface the JavaScript side addresses as ``otkoBridge``."""
+
+    #: (kind, entity id, additive modifier held) where kind is node|element.
+    picked = Signal(str, int, bool)
+    #: World-space coordinates of a clicked grid intersection.
+    snapClicked = Signal(float, float, float)
+
+    @Slot(str, int, bool)
+    def onPicked(self, kind: str, entity_id: int, additive: bool) -> None:
+        """JS entry point: a model entity (or snap target) was clicked."""
+        self.picked.emit(kind, int(entity_id), bool(additive))
+
+    @Slot(float, float, float)
+    def onSnapClicked(self, x: float, y: float, z: float) -> None:
+        """JS entry point: a grid-intersection target was clicked."""
+        self.snapClicked.emit(float(x), float(y), float(z))

src/otko/views/canvas_plotly/html.py

@@ -0,0 +1,276 @@
+"""HTML/JS runtime for the Plotly canvas.
+
+The page is materialised once per process into a temp directory and loaded
+from ``file://``: the plotly.js bundle is ~5 MB, which is past
+``QWebEngineView.setHtml``'s data-URL limit, and writing it to disk also lets
+the browser cache it across figure updates.
+
+The JS side exposes three entry points to Python (called via
+``QWebEnginePage.runJavaScript``):
+
+- ``otkoUpdate(payloadJson)`` — replace data + layout with ``Plotly.react``,
+  which diffs client-side and leaves the interactive camera untouched.
+- ``otkoSetCamera(cameraJson)`` — apply a camera alone (view presets,
+  parallel-projection toggle).
+- ``otkoSetSnapEnabled(bool)`` — arm/disarm the hover snap-target preview.
+
+Clicks travel the other way through the ``otkoBridge`` QWebChannel object:
+entity picks carry the trace ``meta.kind`` and the point ``customdata``.
+"""
+
+from __future__ import annotations
+
+import atexit
+import shutil
+import tempfile
+from pathlib import Path
+
+#: Materialised runtime directory (plotly.min.js + index.html + qwebchannel.js).
+_ASSETS_DIR: Path | None = None
+
+# ``qrc:///qtwebchannel/qwebchannel.js`` is served by QtWebEngine's internal
+# resource scheme; it is not reachable through QFile.
+_PAGE = """<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8"/>
+<script src="qrc:///qtwebchannel/qwebchannel.js"></script>
+<style>
+  html, body { height: 100%; margin: 0; padding: 0; overflow: hidden; background: #f5f7fb; }
+  #plot { width: 100%; height: 100%; }
+  .modebar { display: none !important; }
+</style>
+</head>
+<body>
+<div id="plot"></div>
+<script src="plotly.min.js"></script>
+<script>
+(function () {
+  var bridge = null;
+  var snapEnabled = false;
+  var handlersReady = false;
+  var hoverIndex = -1;
+
+  var config = {
+    responsive: true,
+    displayModeBar: false,
+    scrollZoom: true,
+    doubleClick: false,
+    displaylogo: false
+  };
+
+  new QWebChannel(qt.webChannelTransport, function (channel) {
+    bridge = channel.objects.otkoBridge;
+    window.otkoBridge = bridge;
+  });
+
+  function kindOf(point) {
+    var data = point && point.data;
+    return data && data.meta ? data.meta.kind : null;
+  }
+
+  function findHover() {
+    var gd = document.getElementById('plot');
+    hoverIndex = -1;
+    if (!gd || !gd.data) return;
+    for (var i = 0; i < gd.data.length; i++) {
+      var meta = gd.data[i] && gd.data[i].meta;
+      if (meta && meta.kind === 'hover') { hoverIndex = i; break; }
+    }
+    // A freshly pushed figure has an empty hover marker again.
+    hoverShown = false;
+    hoverPoint = null;
+  }
+
+  // --- snap-target marker ------------------------------------------------
+  // Updated synchronously but only when the visual state actually changes.
+  // Restyling on every hover/unhover made the plot redraw per mouse move,
+  // which re-fired hover and ended in a runaway redraw loop (RangeError:
+  // Maximum call stack size exceeded, frozen canvas). Timers were avoided
+  // deliberately: WebEngine throttles them to ~1 s when the page is not
+  // compositing, which made the preview lag.
+  var hoverShown = false;
+  var hoverPoint = null;
+
+  function samePoint(a, b) {
+    return !!a && !!b && a[0] === b[0] && a[1] === b[1] && a[2] === b[2];
+  }
+
+  function setHoverMarker(point) {
+    var want = snapEnabled ? point : null;
+    if (samePoint(want, hoverPoint) && !!want === hoverShown) return;
+    if (!want && !hoverShown) { hoverPoint = null; return; }
+    hoverPoint = want;
+    hoverShown = !!want;
+    if (hoverIndex < 0) return;
+    var x = want ? [want[0]] : [];
+    var y = want ? [want[1]] : [];
+    var z = want ? [want[2]] : [];
+    Plotly.restyle('plot', { x: [x], y: [y], z: [z] }, [hoverIndex]);
+  }
+
+  // --- view preservation -------------------------------------------------
+  // `Plotly.react` resets any scene attribute the incoming layout omits, so a
+  // non-framing update carries the live camera/ranges forward. Only plain,
+  // validated copies are merged: the raw `_fullLayout` objects are owned by
+  // plotly (react mutates them in place), and injecting an undefined value
+  // would make every later react fail, wedging the plot.
+  function toPlain(value) {
+    try { return JSON.parse(JSON.stringify(value)); } catch (err) { return null; }
+  }
+
+  function currentView() {
+    var gd = document.getElementById('plot');
+    var scene = gd && gd._fullLayout ? gd._fullLayout.scene : null;
+    if (!scene) return null;
+    var view = { ranges: {} };
+    var camera = toPlain(scene.camera);
+    if (camera && camera.eye && camera.center) {
+      view.camera = { eye: camera.eye, center: camera.center, up: camera.up };
+      if (camera.projection) view.camera.projection = camera.projection;
+    }
+    ['xaxis', 'yaxis', 'zaxis'].forEach(function (axis) {
+      var source = scene[axis];
+      if (!source || !Array.isArray(source.range) || source.range.length !== 2) return;
+      var low = Number(source.range[0]);
+      var high = Number(source.range[1]);
+      if (isFinite(low) && isFinite(high) && high > low) view.ranges[axis] = [low, high];
+    });
+    return view;
+  }
+
+  function mergeView(fig, view) {
+    try {
+      if (!view) return;
+      fig.layout.scene = fig.layout.scene || {};
+      if (view.camera) fig.layout.scene.camera = view.camera;
+      Object.keys(view.ranges).forEach(function (axis) {
+        fig.layout.scene[axis] = fig.layout.scene[axis] || {};
+        fig.layout.scene[axis].range = view.ranges[axis];
+        fig.layout.scene[axis].autorange = false;
+      });
+    } catch (err) {
+      // Never let view preservation block the figure update.
+      if (window.console) console.warn('otko: view merge skipped', err);
+    }
+  }
+
+  // --- update queue ------------------------------------------------------
+  // One react at a time, latest payload wins. Overlapping reacts on the same
+  // graph div are what left the plot unresponsive before.
+  var pendingUpdate = null;
+  var reactBusy = false;
+
+  function runUpdate() {
+    if (reactBusy || !pendingUpdate) return;
+    var job = pendingUpdate;
+    pendingUpdate = null;
+    reactBusy = true;
+
+    var fig;
+    try {
+      fig = JSON.parse(job.payload);
+    } catch (err) {
+      reactBusy = false;
+      if (window.console) console.error('otko: bad payload', err);
+      return;
+    }
+    mergeView(fig, job.preserve ? currentView() : null);
+
+    Plotly.react('plot', fig.data, fig.layout, config).then(function () {
+      findHover();
+      installHandlers();
+    }, function (err) {
+      if (window.console) console.error('otko: react failed', err);
+    }).then(function () {
+      reactBusy = false;
+      runUpdate();
+    });
+  }
+
+  window.otkoUpdate = function (payloadJson, preserveView) {
+    pendingUpdate = { payload: payloadJson, preserve: !!preserveView };
+    runUpdate();
+  };
+
+  function installHandlers() {
+    if (handlersReady) return;
+    var gd = document.getElementById('plot');
+    if (!gd || !gd.on) return;
+    handlersReady = true;
+
+    gd.on('plotly_click', function (ev) {
+      var pts = ev.points || [];
+      if (!pts.length || !bridge) return;
+      var p = pts[0];
+      var kind = kindOf(p);
+      var additive = !!(ev.event && (ev.event.shiftKey || ev.event.ctrlKey || ev.event.metaKey));
+      if (kind === 'node' || kind === 'element') {
+        bridge.onPicked(kind, p.customdata, additive);
+      } else if (kind === 'snap') {
+        var c = p.customdata;
+        bridge.onSnapClicked(c[0], c[1], c[2]);
+      }
+    });
+
+    gd.on('plotly_hover', function (ev) {
+      if (!snapEnabled) return;
+      var pts = ev.points || [];
+      if (!pts.length) return;
+      var p = pts[0];
+      if (kindOf(p) !== 'snap') { setHoverMarker(null); return; }
+      var c = p.customdata;
+      if (c) setHoverMarker([c[0], c[1], c[2]]);
+    });
+
+    gd.on('plotly_unhover', function () {
+      // Nothing can be visible unless snapping is armed, so stay out of the
+      // redraw path entirely for ordinary mouse movement.
+      if (!snapEnabled) return;
+      setHoverMarker(null);
+    });
+  }
+
+  window.otkoSetCamera = function (cameraJson) {
+    Plotly.relayout('plot', { 'scene.camera': JSON.parse(cameraJson) });
+  };
+
+  window.otkoSetSnapEnabled = function (on) {
+    snapEnabled = !!on;
+    if (!snapEnabled) setHoverMarker(null);
+  };
+})();
+</script>
+</body>
+</html>
+"""
+
+
+def runtime_url() -> str:
+    """Ensure the JS runtime is on disk and return the page's file path.
+
+    Called once per :class:`~otko.views.canvas_plotly.PlotlyCanvas`; the
+    directory is reused and removed at interpreter exit.
+    """
+    global _ASSETS_DIR
+    directory = _ensure_dir()
+    index = directory / "index.html"
+    if not index.exists():
+        (directory / "index.html").write_text(_PAGE, encoding="utf-8")
+        (directory / "plotly.min.js").write_text(_plotly_js(), encoding="utf-8")
+    return str(index)
+
+
+def _ensure_dir() -> Path:
+    global _ASSETS_DIR
+    if _ASSETS_DIR is None:
+        _ASSETS_DIR = Path(tempfile.mkdtemp(prefix="otko-plotly-"))
+        atexit.register(shutil.rmtree, _ASSETS_DIR, True)
+    return _ASSETS_DIR
+
+
+def _plotly_js() -> str:
+    """The offline plotly.js bundle shipped inside the ``plotly`` package."""
+    from plotly.offline import get_plotlyjs
+
+    return get_plotlyjs()

src/otko/views/canvas_plotly/plotly_canvas.py

@@ -0,0 +1,417 @@
+"""Plotly-backed 3D canvas.
+
+A ``QWebEngineView`` hosting plotly.js, driven over ``QWebChannel``. It
+implements the same public surface as :class:`otko.views.canvas3d.ModelCanvas`
+(signals, selection, working plane, view presets, display toggles) so
+``MainWindow`` can swap the two at runtime.
+
+Update strategy: figures are pushed with ``Plotly.react``. Because react
+resets any scene attribute the incoming layout omits, a non-framing push is
+flagged ``preserveView`` and the JS side carries the live camera and axis
+ranges forward — so re-rendering on a model edit or selection change never
+yanks the camera the user is orbiting. Only an explicit re-frame (new
+project, view preset, ``reset_camera``, style change) sends the computed
+camera plus opstool-style padded axis ranges.
+"""
+
+from __future__ import annotations
+
+import json
+import math
+from collections.abc import Callable
+from dataclasses import replace
+from typing import Any
+
+from PySide6.QtCore import QObject, QUrl, Signal
+from PySide6.QtWebChannel import QWebChannel
+from PySide6.QtWebEngineCore import QWebEnginePage
+from PySide6.QtWebEngineWidgets import QWebEngineView
+from PySide6.QtWidgets import QVBoxLayout, QWidget
+
+from otko.views.canvas3d.model_renderer import RendererMode
+from otko.views.canvas3d.selection import SelectionState
+from otko.views.canvas3d.style import RenderStyle
+from otko.views.canvas_base import CanvasCapabilities
+from otko.views.canvas_plotly import html as _html
+from otko.views.canvas_plotly.bridge import _Bridge
+from otko.views.canvas_plotly.trace_builder import (
+    PlotlyTraceBuilder,
+    Scene,
+    SceneOptions,
+)
+
+#: View preset directions (unit-ish vectors from the scene centre to the eye).
+_VIEW_DIRECTIONS = {
+    "iso": (1.0, 1.0, 0.8),
+    "xy": (0.0, 0.0, 1.0),
+    "xz": (0.0, -1.0, 0.0),
+    "yz": (1.0, 0.0, 0.0),
+}
+
+
+class _LoggingPage(QWebEnginePage):
+    """A web page that forwards the JS console to the app's console dock.
+
+    A plotly failure inside the page is otherwise invisible from Python (the
+    ``react`` promise just rejects), which makes a frozen canvas very hard to
+    diagnose.
+    """
+
+    def __init__(self, log: Callable[[str, str], None], parent: QObject | None = None) -> None:
+        super().__init__(parent)
+        self._log = log
+
+    def javaScriptConsoleMessage(
+        self,
+        level: QWebEnginePage.JavaScriptConsoleMessageLevel,
+        message: str,
+        line_number: int,
+        source_id: str,
+    ) -> None:
+        names = {
+            QWebEnginePage.JavaScriptConsoleMessageLevel.ErrorMessageLevel: "error",
+            QWebEnginePage.JavaScriptConsoleMessageLevel.WarningMessageLevel: "warning",
+        }
+        severity = names.get(level, "info")
+        source = source_id.rsplit("/", 1)[-1] or "plotly"
+        self._log(f"[web] {source}:{line_number} {message}", severity)
+
+
+class _CameraShim:
+    """Mimics ``canvas.camera.parallel_projection`` as consumed elsewhere."""
+
+    def __init__(self, canvas: PlotlyCanvas) -> None:
+        self._canvas = canvas
+
+    @property
+    def parallel_projection(self) -> bool:
+        return self._canvas._parallel
+
+    @parallel_projection.setter
+    def parallel_projection(self, value: bool) -> None:
+        self._canvas.set_parallel_projection(bool(value))
+
+
+class _PlotlyRendererFacade:
+    """Stand-in for ``ModelRenderer`` covering the calls made on ``_renderer``.
+
+    ``RenderControls`` and ``DockManager`` reach into ``canvas._renderer`` for
+    ``render`` / ``set_mode`` / ``_mode``; this keeps those code paths
+    backend-agnostic.
+    """
+
+    def __init__(self, canvas: PlotlyCanvas) -> None:
+        self._canvas = canvas
+
+    @property
+    def _mode(self) -> RendererMode:
+        return self._canvas._mode
+
+    @property
+    def _project(self) -> Any:
+        return self._canvas._project
+
+    @property
+    def _working_plane(self) -> tuple[str, float] | None:
+        return self._canvas._working_plane
+
+    def render(self, project: Any) -> None:
+        self._canvas.set_project(project)
+
+    def set_mode(self, mode: RendererMode, deformation: Any = None) -> None:
+        self._canvas.set_mode(mode, deformation)
+
+    def set_working_plane(self, plane: tuple[str, float] | None) -> None:
+        if plane is None:
+            self._canvas.set_working_plane(None, 0.0)
+        else:
+            self._canvas.set_working_plane(plane[0], plane[1])
+
+    def set_show_section_extrusions(self, on: bool) -> None:
+        self._canvas.set_show_section_extrusions(on)
+
+    def set_show_local_axes(self, on: bool) -> None:
+        self._canvas.set_show_local_axes(on)
+
+    def set_display_options(self, *, show_node_labels: bool, show_element_labels: bool) -> None:
+        self._canvas.set_display_options(
+            show_node_labels=show_node_labels,
+            show_element_labels=show_element_labels,
+        )
+
+
+class PlotlyCanvas(QWidget):
+    """The central 3D viewport, rendered by plotly.js in a web view."""
+
+    nodePicked = Signal(int)
+    elementPicked = Signal(int)
+    emptyClicked = Signal(float, float, float)
+
+    #: Force diagrams and off-screen video capture are not implemented on
+    #: this backend yet (both are PyVista-specific today).
+    capabilities = CanvasCapabilities(diagrams=False, animation_export=False)
+
+    def __init__(
+        self,
+        parent: QWidget | None = None,
+        style: RenderStyle | None = None,
+        selection: SelectionState | None = None,
+        log: Callable[[str, str], None] | None = None,
+    ) -> None:
+        super().__init__(parent)
+        self._log = log
+        self._style = style or RenderStyle()
+        self.selection = selection or SelectionState(self)
+        self._builder = PlotlyTraceBuilder(self._style)
+        self._project: Any = None
+        self._scene = Scene(data=[], layout={})
+        self._options = SceneOptions()
+        self._mode = RendererMode.MODEL
+        self._parallel = False
+        self._view_preset = "iso"
+        self._snap_enabled = False
+        self._default_selection_enabled = True
+        self._working_plane: tuple[str, float] | None = None
+        self._camera = _CameraShim(self)
+        self._framing_dirty = True
+        self._renderer = _PlotlyRendererFacade(self)
+        self._ready = False
+
+        self._build_ui()
+        self.selection.selectionChanged.connect(self._on_selection_changed)
+
+    # ── construction ─────────────────────────────────────────────────
+    def _build_ui(self) -> None:
+        layout = QVBoxLayout(self)
+        layout.setContentsMargins(0, 0, 0, 0)
+        self._web = QWebEngineView(self)
+        if self._log is not None:
+            self._web.setPage(_LoggingPage(self._log, self._web))
+        self._channel = QWebChannel(self._web.page())
+        self._bridge = _Bridge(self)
+        self._channel.registerObject("otkoBridge", self._bridge)
+        self._web.page().setWebChannel(self._channel)
+        self._web.loadFinished.connect(self._on_loaded)
+        self._bridge.picked.connect(self._on_picked)
+        self._bridge.snapClicked.connect(self._on_snap_clicked)
+        layout.addWidget(self._web)
+        self._web.load(QUrl.fromLocalFile(_html.runtime_url()))
+
+    def _on_loaded(self, ok: bool) -> None:
+        if not ok:
+            return
+        self._ready = True
+        self._push_scene()
+        self._eval(f"window.otkoSetSnapEnabled({_js_bool(self._snap_enabled)})")
+
+    # ── public API (mirrors ModelCanvas) ─────────────────────────────
+    def show_project(self, project: Any) -> None:
+        """Render (or clear) a project; frame the camera when nodes exist."""
+        self.set_project(project)
+        if project is not None and project.nodes:
+            self._view_preset = "iso"
+            self._framing_dirty = True
+        self.render()
+
+    def clear_model(self) -> None:
+        """Remove the model but keep the view/selection machinery."""
+        self.selection.clear()
+        self.set_project(None)
+        self.render()
+
+    def render(self, *args: Any, **kwargs: Any) -> None:
+        """Rebuild the figure and hand it to plotly.js.
+
+        ``QWidget.render`` is overloaded for painting into a target; those
+        calls are forwarded untouched so the widget stays well-behaved, and
+        only the no-argument canvas idiom (shared with ``ModelCanvas``)
+        triggers a figure push.
+        """
+        if args or kwargs:
+            super().render(*args, **kwargs)
+            return
+        self._push_scene()
+
+    def set_project(self, project: Any) -> None:
+        """Bind a project without rendering (used by the ``_renderer`` facade)."""
+        self._project = project
+
+    def set_mode(self, mode: RendererMode, deformation: Any = None) -> None:
+        """Set MODEL / DEFORMED / MODAL plus the displacement source to apply."""
+        self._mode = mode
+        self._options = replace(self._options, deformation=deformation)
+
+    def set_parallel_projection(self, on: bool) -> None:
+        self._parallel = bool(on)
+        self._framing_dirty = True
+
+    def reset_camera(self) -> None:
+        self._view_preset = "iso"
+        self._framing_dirty = True
+        self.render()
+
+    def view_isometric(self) -> None:
+        self._view_preset = "iso"
+        self._framing_dirty = True
+        self.render()
+
+    def view_xy(self) -> None:
+        """Top view: the eye sits on +Z looking down."""
+        self._view_preset = "xy"
+        self._framing_dirty = True
+        self.render()
+
+    def view_xz(self) -> None:
+        """Front view: the eye sits on -Y."""
+        self._view_preset = "xz"
+        self._framing_dirty = True
+        self.render()
+
+    def view_yz(self) -> None:
+        """Right view: the eye sits on +X."""
+        self._view_preset = "yz"
+        self._framing_dirty = True
+        self.render()
+
+    # ── working plane ────────────────────────────────────────────────
+    def set_working_plane(self, plane: str | None, offset: float) -> None:
+        """Filter the grid overlay to the active plan / elevation level."""
+        if plane is None:
+            self._working_plane = None
+        else:
+            if plane not in ("XY", "XZ", "YZ"):
+                raise ValueError(f"Unsupported working plane: {plane!r}")
+            self._working_plane = (plane, float(offset))
+        self._options = replace(self._options, working_plane=self._working_plane)
+        self.render()
+
+    def clear_working_plane(self) -> None:
+        self.set_working_plane(None, 0.0)
+
+    def working_plane_type(self) -> str | None:
+        return self._working_plane[0] if self._working_plane is not None else None
+
+    def working_plane_offset(self) -> float | None:
+        return self._working_plane[1] if self._working_plane is not None else None
+
+    # ── display toggles ──────────────────────────────────────────────
+    def set_snap_preview_enabled(self, enabled: bool) -> None:
+        """Toggle the hover snap-target preview (draw tools turn it on)."""
+        self._snap_enabled = bool(enabled)
+        self._eval(f"window.otkoSetSnapEnabled({_js_bool(self._snap_enabled)})")
+
+    def set_show_section_extrusions(self, enabled: bool) -> None:
+        self._options = replace(self._options, show_extrusions=bool(enabled))
+        self.render()
+
+    def set_show_local_axes(self, enabled: bool) -> None:
+        self._options = replace(self._options, show_local_axes=bool(enabled))
+        self.render()
+
+    def set_style(self, style: RenderStyle) -> None:
+        """Swap the visual style and repaint the figure.
+
+        Deliberately *not* a re-frame: colour/opacity are trace props and the
+        background/outline ride the layout, so a style change (including the
+        live preview while dragging a colour) updates without snapping the
+        camera the user has moved.
+        """
+        self._style = style
+        self._builder.set_style(style)
+        self.render()
+
+    def set_display_options(self, *, show_node_labels: bool, show_element_labels: bool) -> None:
+        self._options = replace(
+            self._options,
+            show_node_labels=bool(show_node_labels),
+            show_element_labels=bool(show_element_labels),
+        )
+        self.render()
+
+    def set_default_selection_enabled(self, enabled: bool) -> None:
+        """When False, picks fire signals but do not touch :attr:`selection`."""
+        self._default_selection_enabled = bool(enabled)
+
+    # ── internals ───────────────────────────────────────────────────
+    def _push_scene(self) -> None:
+        """Send the figure to plotly.js.
+
+        ``Plotly.react`` resets any scene attribute the incoming layout omits,
+        which used to snap the camera *and* the padded ranges back on every
+        selection change. So a non-framing push marks itself
+        ``preserveView`` and the JS side carries the live camera/ranges
+        forward; only an explicit re-frame (new project, view preset,
+        ``reset_camera``) sends the computed framing. Style changes are
+        non-framing: they re-colour in place.
+        """
+        self._scene = self._builder.build(self._project, self._options)
+        if not self._ready:
+            # Nothing to push yet; crucially this must come *before* the
+            # framing flag is consumed, or the framing scheduled before
+            # loadFinished would be dropped on the floor.
+            return
+        layout = dict(self._scene.layout)
+        re_framed = self._framing_dirty
+        if re_framed:
+            scene_layout = dict(layout["scene"])
+            scene_layout["camera"] = self._camera_dict(self._scene)
+            for axis, override in self._scene.axis_overrides().items():
+                scene_layout[axis] = {**scene_layout.get(axis, {}), **override}
+            layout["scene"] = scene_layout
+            self._framing_dirty = False
+        payload = json.dumps({"data": self._scene.data, "layout": layout})
+        self._eval(f"window.otkoUpdate({json.dumps(payload)}, {_js_bool(not re_framed)})")
+
+    def _camera_dict(self, scene: Scene) -> dict[str, Any]:
+        cx, cy, cz = scene.center
+        distance = max(scene.diagonal, 1e-6) * 1.6
+        direction = _VIEW_DIRECTIONS.get(self._view_preset, _VIEW_DIRECTIONS["iso"])
+        norm = math.sqrt(sum(component * component for component in direction)) or 1.0
+        eye = (
+            cx + direction[0] / norm * distance,
+            cy + direction[1] / norm * distance,
+            cz + direction[2] / norm * distance,
+        )
+        # Looking straight down the Z axis needs a non-degenerate up vector.
+        up = (0.0, 1.0, 0.0) if self._view_preset == "xy" else (0.0, 0.0, 1.0)
+        return {
+            "eye": {"x": eye[0], "y": eye[1], "z": eye[2]},
+            "center": {"x": cx, "y": cy, "z": cz},
+            "up": {"x": up[0], "y": up[1], "z": up[2]},
+            "projection": {"type": "orthographic" if self._parallel else "perspective"},
+        }
+
+    def _eval(self, js: str) -> None:
+        if self._ready:
+            self._web.page().runJavaScript(js)
+
+    def _on_selection_changed(self, nodes: frozenset[int], elements: frozenset[int]) -> None:
+        self._options = replace(
+            self._options,
+            selection_nodes=frozenset(nodes),
+            selection_elements=frozenset(elements),
+        )
+        self.render()
+
+    def _on_picked(self, kind: str, entity_id: int, additive: bool) -> None:
+        if kind == "node":
+            if self._default_selection_enabled:
+                if additive:
+                    self.selection.toggle_node(entity_id)
+                else:
+                    self.selection.select_node(entity_id)
+            self.nodePicked.emit(entity_id)
+        elif kind == "element":
+            if self._default_selection_enabled:
+                if additive:
+                    self.selection.toggle_element(entity_id)
+                else:
+                    self.selection.select_element(entity_id)
+            self.elementPicked.emit(entity_id)
+
+    def _on_snap_clicked(self, x: float, y: float, z: float) -> None:
+        self.emptyClicked.emit(float(x), float(y), float(z))
+
+
+def _js_bool(value: bool) -> str:
+    return "true" if value else "false"

src/otko/views/dialogs/__init__.py

@@ -34,6 +34,7 @@ from otko.views.dialogs.move import MoveDialog
 from otko.views.dialogs.path_time_series import PathTimeSeriesDialog
 from otko.views.dialogs.pattern_loads import PatternLoadsDialog
 from otko.views.dialogs.plain_pattern import PlainPatternDialog
+from otko.views.dialogs.plot_properties import PlotPropertiesDialog
 from otko.views.dialogs.quick_grid_lines import QuickGridLinesDialog
 from otko.views.dialogs.quick_guide import QuickGuideDialog
 from otko.views.dialogs.replicate import ReplicateDialog
@@ -69,6 +70,7 @@ __all__ = [
     "PathTimeSeriesDialog",
     "PatternLoadsDialog",
     "PlainPatternDialog",
+    "PlotPropertiesDialog",
     "QuickGridLinesDialog",
     "QuickGuideDialog",
     "ReplicateDialog",

src/otko/views/dialogs/plot_properties.py

@@ -0,0 +1,150 @@
+"""Plot Properties dialog — the user-facing subset of :class:`RenderStyle`.
+
+The field table lives in :mod:`otko.views.canvas3d.style` (``STYLE_FIELDS``),
+so adding an editable property is a one-line change there plus a default on
+the dataclass. Edits are emitted through :attr:`PlotPropertiesDialog.changed`
+so the canvas can preview them live; ``style()`` returns the composed result.
+"""
+
+from __future__ import annotations
+
+from PySide6.QtCore import Signal
+from PySide6.QtGui import QColor
+from PySide6.QtWidgets import (
+    QCheckBox,
+    QColorDialog,
+    QDialog,
+    QDialogButtonBox,
+    QDoubleSpinBox,
+    QFormLayout,
+    QLabel,
+    QPushButton,
+    QSpinBox,
+    QVBoxLayout,
+    QWidget,
+)
+
+from otko.views.canvas3d.style import STYLE_FIELDS, RenderStyle
+
+
+class _ColorButton(QPushButton):
+    """A swatch that opens :class:`QColorDialog` and reports the hex value."""
+
+    colorChanged = Signal(str)
+
+    def __init__(self, color: str, parent: QWidget | None = None) -> None:
+        super().__init__(parent)
+        self._color = color
+        self.setFixedWidth(96)
+        self._refresh()
+        self.clicked.connect(self._pick)
+
+    def color(self) -> str:
+        """Current colour as a ``#rrggbb`` string."""
+        return self._color
+
+    def set_color(self, color: str) -> None:
+        """Set the swatch colour (does not emit :attr:`colorChanged`)."""
+        self._color = color
+        self._refresh()
+
+    def _refresh(self) -> None:
+        self.setText(self._color.upper())
+        self.setStyleSheet(f"background-color: {self._color}; color: {self._text_color()};")
+
+    def _text_color(self) -> str:
+        color = QColor(self._color)
+        luminance = 0.299 * color.red() + 0.587 * color.green() + 0.114 * color.blue()
+        return "#000000" if luminance > 140 else "#ffffff"
+
+    def _pick(self) -> None:
+        chosen = QColorDialog.getColor(QColor(self._color), self, "Select colour")
+        if chosen.isValid():
+            self.set_color(chosen.name())
+            self.colorChanged.emit(self._color)
+
+
+class PlotPropertiesDialog(QDialog):
+    """Modeless-free editor for the persisted plot style."""
+
+    #: Emitted with the composed :class:`RenderStyle` on every edit.
+    changed = Signal(object)
+
+    def __init__(self, style: RenderStyle, parent: QWidget | None = None) -> None:
+        super().__init__(parent)
+        self.setWindowTitle("Plot Properties")
+        self._base = style
+        self._colors: dict[str, _ColorButton] = {}
+        self._numbers: dict[str, QDoubleSpinBox | QSpinBox] = {}
+        self._checks: dict[str, QCheckBox] = {}
+
+        layout = QVBoxLayout(self)
+        form = QFormLayout()
+        for name, label, kind in STYLE_FIELDS:
+            widget = self._build_widget(name, getattr(style, name), kind)
+            form.addRow(QLabel(f"{label}:"), widget)
+        layout.addLayout(form)
+
+        buttons = QDialogButtonBox(
+            QDialogButtonBox.StandardButton.Ok | QDialogButtonBox.StandardButton.Cancel
+        )
+        reset = buttons.addButton("Reset to Defaults", QDialogButtonBox.ButtonRole.ResetRole)
+        assert reset is not None
+        reset.clicked.connect(self._reset)
+        buttons.accepted.connect(self.accept)
+        buttons.rejected.connect(self.reject)
+        layout.addWidget(buttons)
+
+    # ── public ──────────────────────────────────────────────────────
+    def plot_style(self) -> RenderStyle:
+        """The composed style for the current widget values.
+
+        Named ``plot_style`` (not ``style``) because :meth:`QWidget.style`
+        already exists and must keep its Qt meaning.
+        """
+        updates: dict[str, object] = {name: w.color() for name, w in self._colors.items()}
+        updates.update({name: w.value() for name, w in self._numbers.items()})
+        updates.update({name: w.isChecked() for name, w in self._checks.items()})
+        return self._base.with_updates(**updates)
+
+    # ── internals ───────────────────────────────────────────────────
+    def _build_widget(self, name: str, value: object, kind: str) -> QWidget:
+        if kind == "color":
+            button = _ColorButton(str(value))
+            button.colorChanged.connect(self._emit_changed)
+            self._colors[name] = button
+            return button
+        if kind == "float":
+            spin = QDoubleSpinBox()
+            spin.setRange(0.0, 1.0)
+            spin.setSingleStep(0.05)
+            spin.setDecimals(2)
+            spin.setValue(float(value))  # type: ignore[arg-type]
+            spin.valueChanged.connect(self._emit_changed)
+            self._numbers[name] = spin
+            return spin
+        if kind == "bool":
+            check = QCheckBox()
+            check.setChecked(bool(value))
+            check.toggled.connect(self._emit_changed)
+            self._checks[name] = check
+            return check
+        int_spin = QSpinBox()
+        int_spin.setRange(6, 32)
+        int_spin.setValue(int(value))  # type: ignore[call-overload]
+        int_spin.valueChanged.connect(self._emit_changed)
+        self._numbers[name] = int_spin
+        return int_spin
+
+    def _emit_changed(self, *_args: object) -> None:
+        self.changed.emit(self.plot_style())
+
+    def _reset(self) -> None:
+        defaults = RenderStyle()
+        for name, button in self._colors.items():
+            button.set_color(getattr(defaults, name))
+        for name, spin in self._numbers.items():
+            spin.setValue(getattr(defaults, name))
+        for name, check in self._checks.items():
+            check.setChecked(getattr(defaults, name))
+        self._emit_changed()

src/otko/views/dock_manager.py

@@ -210,6 +210,14 @@ class DockManager:
         because the off-screen renderer must be touched from the GUI
         thread (VTK's Qt-backed render window isn't thread-safe).
         """
+        if not self._canvas.capabilities.animation_export:
+            QMessageBox.information(
+                self,
+                "Export Mode Shape Animation",
+                "Animation export needs the PyVista backend "
+                "(Options → Canvas Backend → PyVista (Native)).",
+            )
+            return
         from PySide6.QtWidgets import QFileDialog
 
         path, _sel = QFileDialog.getSaveFileName(
@@ -260,6 +268,14 @@ class DockManager:
                 "Run a Static analysis first; force diagrams visualise its element-force output.",
             )
             return
+        if not self._canvas.capabilities.diagrams:
+            QMessageBox.information(
+                self,
+                "Force Diagram",
+                "Force diagrams are not available on the Plotly backend yet.\n\n"
+                "Switch back via Options → Canvas Backend → PyVista (Native).",
+            )
+            return
         self._tear_down_post_dock()
 
         # Pick the component with the largest abs_max as the initial choice
@@ -364,6 +380,14 @@ class DockManager:
                 "Run a Transient (time-history) analysis first.",
             )
             return
+        if not self._canvas.capabilities.animation_export:
+            QMessageBox.information(
+                self,
+                "Export Time-History Animation",
+                "Animation export needs the PyVista backend "
+                "(Options → Canvas Backend → PyVista (Native)).",
+            )
+            return
         from pathlib import Path
 
         from PySide6.QtWidgets import QFileDialog, QInputDialog

src/otko/views/main_window.py

@@ -13,16 +13,21 @@ The ``projectChanged`` / ``modelMutated`` signals live on the
 
 from __future__ import annotations
 
+import json
 from pathlib import Path
+from typing import Any
 
 from PySide6.QtCore import QSettings
 from PySide6.QtGui import QCloseEvent, QIcon
-from PySide6.QtWidgets import QMainWindow, QMessageBox
+from PySide6.QtWidgets import QMainWindow, QMessageBox, QStackedWidget
 
 from otko.viewmodels import AnalysisRunner, ProjectViewModel
 from otko.views.action_handlers import ActionHandlers
 from otko.views.canvas3d import ModelCanvas
 from otko.views.canvas3d.diagram_renderer import DiagramRenderer
+from otko.views.canvas3d.selection import SelectionState
+from otko.views.canvas3d.style import RenderStyle
+from otko.views.canvas_base import plotly_available
 from otko.views.dock_manager import DockManager
 from otko.views.menu_builder import MenuBuilder
 from otko.views.render_controls import RenderControls
@@ -34,6 +39,9 @@ from otko.views.tools import (
     ToolController,
 )
 
+#: Canvas backends, in menu order.
+CANVAS_BACKENDS = ("pyvista", "plotly")
+
 
 class MainWindow(
     QMainWindow,
@@ -44,7 +52,7 @@ class MainWindow(
 ):
     """Top-level application shell."""
 
-    def __init__(self) -> None:
+    def __init__(self, settings: QSettings | None = None) -> None:
         super().__init__()
         self.setWindowTitle("OTKO")
         self.setWindowIcon(
@@ -60,12 +68,22 @@ class MainWindow(
         self._show_node_labels = False
         self._show_element_labels = False
 
-        self._build_central_canvas()
-        self._tool_controller = ToolController(self._canvas, self._vm, self)
-        self._select_tool = SelectTool(self._canvas, self._vm, self)
-        self._draw_frame_tool: DrawFrameTool | None = None  # lazy-created on activation
-        self._draw_node_tool: DrawNodeTool | None = None
-        self._draw_truss_tool: DrawTrussTool | None = None
+        # One selection state shared by every canvas backend so a live swap
+        # keeps the current selection (canvas, tree, table, properties dock).
+        self._selection = SelectionState(self)
+
+        self._settings = settings if settings is not None else QSettings("OTKO", "OTKO")
+        self._canvas_backend = self._stored_canvas_backend()
+        #: Shared, user-editable visual style (both backends read it).
+        self._style = self._load_plot_style()
+        self._canvases: dict[str, Any] = {}
+        self._diagram_renderers: dict[str, DiagramRenderer] = {}
+        # A stack holds every backend's canvas: swapping is just
+        # setCurrentWidget, so no backend widget is ever destroyed mid-session.
+        self._canvas_stack = QStackedWidget(self)
+        self.setCentralWidget(self._canvas_stack)
+        self._activate_canvas(self._canvas_backend)
+        self._build_tooling()
 
         self._build_docks()
         self._build_actions()
@@ -77,19 +95,164 @@ class MainWindow(
         self._refresh_action_enablement()
         self.restore_layout()
 
-    # ── construction ─────────────────────────────────────────────────
-    def _build_central_canvas(self) -> None:
-        self._canvas = ModelCanvas(self)
-        self.setCentralWidget(self._canvas)
+    # ── canvas backend ───────────────────────────────────────────────
+    def _stored_canvas_backend(self) -> str:
+        """The backend chosen last session, falling back to PyVista."""
+        stored = self._settings.value("canvas/backend", "pyvista")
+        name = str(stored) if stored is not None else "pyvista"
+        if name == "plotly" and plotly_available():
+            return "plotly"
+        return "pyvista"
+
+    def _ensure_canvas(self, backend: str) -> Any:
+        """Create a backend's canvas on first use and cache it.
+
+        Both canvases are kept alive and swapped with ``setCentralWidget``
+        rather than destroyed: tearing a VTK render window down mid-session
+        leaves dangling make-current callbacks, and the widgets are cheap to
+        retain once built.
+        """
+        canvas = self._canvases.get(backend)
+        if canvas is not None:
+            return canvas
+        if backend == "plotly":
+            # Imported lazily so a plotly-less install still boots PyVista.
+            from otko.views.canvas_plotly import PlotlyCanvas
+
+            canvas = PlotlyCanvas(
+                self,
+                style=self._style,
+                selection=self._selection,
+                log=self._log_canvas_message,
+            )
+        else:
+            canvas = ModelCanvas(self, style=self._style, selection=self._selection)
+        self._canvases[backend] = canvas
+        return canvas
+
+    def _activate_canvas(self, backend: str) -> None:
+        """Make the backend's canvas the central widget and rewire overlays."""
+        if backend not in CANVAS_BACKENDS:
+            backend = "pyvista"
+        canvas = self._ensure_canvas(backend)
+        if self._canvas_stack.indexOf(canvas) < 0:
+            self._canvas_stack.addWidget(canvas)
+        self._canvas_stack.setCurrentWidget(canvas)
+        self._canvas = canvas
+        self._canvas_backend = backend
+        if backend == "pyvista":
+            renderer = self._diagram_renderers.get("pyvista")
+            if renderer is None:
                 # Diagram overlay paints onto the same plotter as the model.
-        self._diagram_renderer = DiagramRenderer(self._canvas)
+                renderer = DiagramRenderer(canvas)
+                self._diagram_renderers["pyvista"] = renderer
+            self._diagram_renderer = renderer
+        else:
+            self._diagram_renderer = None
+
+    def canvas_backend(self) -> str:
+        """Name of the active canvas backend (``"pyvista"`` or ``"plotly"``)."""
+        return self._canvas_backend
+
+    def _log_canvas_message(self, message: str, severity: str = "info") -> None:
+        """Route a JS console line from the Plotly page to the Console dock.
+
+        The dock may not exist yet while canvases are being constructed, so
+        the lookup is guarded.
+        """
+        dock = getattr(self, "_console_dock", None)
+        if dock is not None:
+            dock.log(message, severity=severity)
+
+    # ── plot style ───────────────────────────────────────────────────
+    def _load_plot_style(self) -> RenderStyle:
+        """Style from the last session, ignoring anything unrecognised."""
+        raw = self._settings.value("plot/props")
+        if not raw:
+            return RenderStyle()
+        try:
+            data = json.loads(str(raw))
+        except ValueError:
+            return RenderStyle()
+        if not isinstance(data, dict):
+            return RenderStyle()
+        allowed = {name: value for name, value in data.items() if name in RenderStyle.EDITABLE}
+        try:
+            return RenderStyle(**allowed)
+        except (TypeError, ValueError):
+            return RenderStyle()
+
+    def _save_plot_style(self) -> None:
+        self._settings.setValue("plot/props", json.dumps(self._style.editable_values()))
+
+    def set_plot_style(self, style: RenderStyle, *, persist: bool = True) -> None:
+        """Apply a style to every canvas (live preview when ``persist`` is off)."""
+        self._style = style
+        for canvas in self._canvases.values():
+            canvas.set_style(style)
+        if persist:
+            self._save_plot_style()
+
+    def _build_tooling(self) -> None:
+        """(Re)build the tool controller and tools against the current canvas."""
+        self._tool_controller = ToolController(self._canvas, self._vm, self)
+        self._select_tool = SelectTool(self._canvas, self._vm, self)
+        self._draw_frame_tool: DrawFrameTool | None = None  # lazy-created on activation
+        self._draw_node_tool: DrawNodeTool | None = None
+        self._draw_truss_tool: DrawTrussTool | None = None
+        self._tool_controller.toolChanged.connect(self._on_tool_changed)
+        self._tool_controller.toolChanged.connect(self._sync_tool_actions)
+
+    def swap_canvas_backend(self, backend: str) -> None:
+        """Switch the live canvas backend, preserving model/selection state.
+
+        The camera pose is intentionally re-framed rather than transferred:
+        the two engines have different camera models, and a wrong pose is
+        worse than a clean isometric reset.
+        """
+        if backend not in CANVAS_BACKENDS or backend == self._canvas_backend:
+            return
+        if backend == "plotly" and not plotly_available():
+            return
+
+        project = self._vm.project
+        plane = self._canvas.working_plane_type()
+        offset = self._canvas.working_plane_offset()
+        active_name = getattr(self._tool_controller.active, "name", None)
+
+        self._activate_canvas(backend)
+        self._build_tooling()
+
+        # Project → re-frames the camera; working plane rebuilds the grid.
+        self._canvas.show_project(project)
+        if plane is not None and offset is not None:
+            self._canvas.set_working_plane(plane, offset)
+        # Re-apply the overlays whose state lives on the toolbar actions.
+        self._canvas.set_show_section_extrusions(self._act_show_extruded.isChecked())
+        self._canvas.set_show_local_axes(self._act_show_local_axes.isChecked())
+        self._canvas.set_display_options(
+            show_node_labels=self._show_node_labels,
+            show_element_labels=self._show_element_labels,
+        )
+        # Keep the active tool (and its snap preview) alive across the swap.
+        # Re-entering through the action handlers recreates the tool lazily
+        # against the new canvas.
+        if active_name == "Draw Node":
+            self._on_draw_node_tool()
+        elif active_name == "Draw Frame":
+            self._on_draw_frame_tool()
+        elif active_name == "Draw Truss":
+            self._on_draw_truss_tool()
+
+        self._settings.setValue("canvas/backend", self._canvas_backend)
+        self._sync_canvas_backend_actions()
+        self._refresh_action_enablement()
 
     # ── layout persistence ───────────────────────────────────────────
     def save_layout(self) -> None:
         """Persist window geometry and dock/toolbar state for next launch."""
-        settings = QSettings("OTKO", "OTKO")
-        settings.setValue("geometry", self.saveGeometry())
-        settings.setValue("windowState", self.saveState())
+        self._settings.setValue("geometry", self.saveGeometry())
+        self._settings.setValue("windowState", self.saveState())
 
     def restore_layout(self) -> None:
         """Restore a previously saved geometry/dock layout, if any.
@@ -97,11 +260,10 @@ class MainWindow(
         On first run no values are stored, so the defaults from ``__init__``
         (``resize`` + dock construction) stay in effect.
         """
-        settings = QSettings("OTKO", "OTKO")
-        geometry = settings.value("geometry")
+        geometry = self._settings.value("geometry")
         if geometry is not None:
             self.restoreGeometry(geometry)
-        state = settings.value("windowState")
+        state = self._settings.value("windowState")
         if state is not None:
             self.restoreState(state)
 
@@ -147,6 +309,7 @@ class MainWindow(
         self._act_about.triggered.connect(self._on_about)
         self._act_quick_guide.triggered.connect(self._on_quick_guide)
         self._act_set_units.triggered.connect(self._on_set_units)
+        self._act_plot_properties.triggered.connect(self._on_plot_properties)
 
         # Edit
         self._act_delete.triggered.connect(self._on_delete)
@@ -161,9 +324,12 @@ class MainWindow(
         self._act_tool_draw_node.triggered.connect(self._on_draw_node_tool)
         self._act_tool_draw_frame.triggered.connect(self._on_draw_frame_tool)
         self._act_tool_draw_truss.triggered.connect(self._on_draw_truss_tool)
-        self._tool_controller.toolChanged.connect(self._on_tool_changed)
-        # Keep the Tools toolbar checked state in sync with the controller.
-        self._tool_controller.toolChanged.connect(self._sync_tool_actions)
+        # toolChanged → toolbar/menu sync is wired in _build_tooling(), which
+        # also runs on a canvas-backend swap (then a fresh controller exists).
+
+        # Canvas backend
+        self._act_canvas_pyvista.triggered.connect(lambda: self.swap_canvas_backend("pyvista"))
+        self._act_canvas_plotly.triggered.connect(lambda: self.swap_canvas_backend("plotly"))
 
         # Define
         self._act_grid.triggered.connect(self._on_grid_system)

src/otko/views/menu_builder.py

@@ -192,9 +192,46 @@ class MenuBuilder:
         # No shortcut: F1/F2/F3/F5 are taken by the draw tools and Run.
         self._act_quick_guide = QAction("&Quick Guide", self)
         self._act_set_units = QAction("Set Display &Units…", self)
+        self._act_plot_properties = QAction("&Plot Properties…", self)
+        self._act_plot_properties.setToolTip(
+            "Colours and sizes used by both canvas backends (persisted)."
+        )
+
+        # Canvas backend — exclusive radio pair live-switching the 3D view.
+        from otko.views.canvas_base import plotly_available
+
+        self._canvas_backend_group = QActionGroup(self)
+        self._canvas_backend_group.setExclusive(True)
+        self._act_canvas_pyvista = QAction("&PyVista (Native)", self, checkable=True)
+        self._act_canvas_pyvista.setToolTip(
+            "Native VTK/OpenGL viewport — fastest for large models."
+        )
+        self._act_canvas_plotly = QAction("&Plotly (WebGL)", self, checkable=True)
+        self._act_canvas_plotly.setToolTip(
+            "plotly.js rendered in a WebEngine view — richer graphics, higher overhead."
+        )
+        self._canvas_backend_group.addAction(self._act_canvas_pyvista)
+        self._canvas_backend_group.addAction(self._act_canvas_plotly)
+        if not plotly_available():
+            self._act_canvas_plotly.setEnabled(False)
+            self._act_canvas_plotly.setToolTip(
+                'Install the optional GUI extra (pip install -e ".[gui]") to enable plotly.'
+            )
 
         self._apply_toolbar_icons()
 
+    def _sync_canvas_backend_actions(self) -> None:
+        """Mirror the active canvas backend onto the Options radio pair."""
+        active = getattr(
+            self,
+            "_act_canvas_plotly" if self._canvas_backend == "plotly" else "_act_canvas_pyvista",
+        )
+        self._canvas_backend_group.blockSignals(True)
+        try:
+            active.setChecked(True)
+        finally:
+            self._canvas_backend_group.blockSignals(False)
+
     def _apply_toolbar_icons(self) -> None:
         """Assign icons + self-documenting tips to every toolbar action.
 
@@ -479,6 +516,11 @@ class MenuBuilder:
 
         m_options = mb.addMenu("&Options")
         m_options.addAction(self._act_set_units)
+        m_options.addAction(self._act_plot_properties)
+        m_backend = m_options.addMenu("Canvas &Backend")
+        m_backend.addAction(self._act_canvas_pyvista)
+        m_backend.addAction(self._act_canvas_plotly)
+        self._sync_canvas_backend_actions()
 
         m_help = mb.addMenu("&Help")
         m_help.addAction(self._act_quick_guide)

src/otko/views/render_controls.py

@@ -50,6 +50,23 @@ class RenderControls:
         )
 
     # ── SAP2000-style view switching + working level ───────────────
+    def _on_plot_properties(self) -> None:
+        """Edit the shared visual style.
+
+        Edits preview live on both canvases; Cancel restores the style the
+        dialog opened with, and only OK persists to ``QSettings``.
+        """
+        from otko.views.dialogs import PlotPropertiesDialog
+
+        original = self._style
+        dialog = PlotPropertiesDialog(original, self)
+        dialog.changed.connect(lambda style: self.set_plot_style(style, persist=False))
+        if dialog.exec() == QDialog.DialogCode.Accepted:
+            self.set_plot_style(dialog.plot_style())
+            self._log("Plot properties updated.")
+        else:
+            self.set_plot_style(original, persist=False)
+
     def _on_toggle_parallel(self, on: bool) -> None:
         cam = self._canvas.camera
         cam.parallel_projection = on
@@ -459,6 +476,7 @@ class RenderControls:
         has_pushover = isinstance(self._latest_results, PushoverResults)
         has_rs = isinstance(self._latest_results, ResponseSpectrumResults)
         n_sel = len(self._canvas.selection.nodes)
+        caps = self._canvas.capabilities
         self._act_save.setEnabled(has_project)
         self._act_save_as.setEnabled(has_project)
         self._act_export_opspy.setEnabled(has_project)
@@ -482,9 +500,11 @@ class RenderControls:
         self._act_tool_draw_truss.setEnabled(has_project)
         self._act_show_deformed.setEnabled(has_static)
         self._act_show_mode_shape.setEnabled(has_modal)
-        self._act_show_force_diagram.setEnabled(has_static)
+        # Force diagrams + video export are backend capabilities, not just
+        # data availability (Plotly cannot do either yet).
+        self._act_show_force_diagram.setEnabled(has_static and caps.diagrams)
         self._act_show_time_history.setEnabled(has_transient)
-        self._act_export_th_animation.setEnabled(has_transient)
+        self._act_export_th_animation.setEnabled(has_transient and caps.animation_export)
         self._act_show_hysteresis.setEnabled(has_transient)
         self._act_show_pushover.setEnabled(has_pushover)
         self._act_show_response_spectrum.setEnabled(has_rs)

tests/conftest.py

@@ -11,6 +11,7 @@ domain after every test. The import is inside the teardown so that
 
 from __future__ import annotations
 
+import importlib.util
 import os
 from collections.abc import Iterator
 
@@ -19,6 +20,27 @@ import pytest
 os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
 
 
+@pytest.fixture(autouse=True, scope="session")
+def _isolate_qsettings(tmp_path_factory: pytest.TempPathFactory) -> Iterator[None]:
+    """Redirect ``QSettings`` to a temp dir so tests never touch user state.
+
+    ``MainWindow`` persists preferences (window layout, canvas backend) under
+    ``QSettings("OTKO", "OTKO")``. Without redirection a developer who, say,
+    switched to the Plotly backend would see PyVista-specific GUI tests fail
+    against their real settings. Skipped entirely when Qt is not installed, so
+    the headless job stays free of Qt.
+    """
+    if importlib.util.find_spec("PySide6") is None:
+        yield
+        return
+    from PySide6.QtCore import QSettings
+
+    settings_dir = tmp_path_factory.mktemp("qsettings")
+    for fmt in (QSettings.Format.NativeFormat, QSettings.Format.IniFormat):
+        QSettings.setPath(fmt, QSettings.Scope.UserScope, str(settings_dir))
+    yield
+
+
 @pytest.fixture(autouse=True)
 def _wipe_opensees_domain() -> Iterator[None]:
     """Wipe the OpenseesPy domain after a test if the solver was imported."""

tests/gui/test_canvas_backend_switch.py

@@ -0,0 +1,122 @@
+"""GUI tests for the live canvas-backend switch (PyVista ⇄ Plotly).
+
+QSettings are injected per-test so a swap never leaks into the real user
+configuration or the next test.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+pytest.importorskip("pyvistaqt")
+pytest.importorskip("plotly")
+
+from PySide6.QtCore import QSettings
+
+
+def _make_window(tmp_path, qtbot):  # type: ignore[no-untyped-def]
+    from otko.views.main_window import MainWindow
+
+    settings = QSettings(str(tmp_path / "otko.ini"), QSettings.Format.IniFormat)
+    window = MainWindow(settings=settings)
+    qtbot.addWidget(window)
+    return window
+
+
+@pytest.mark.gui
+def test_default_backend_is_pyvista(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.canvas3d import ModelCanvas
+
+    window = _make_window(tmp_path, qtbot)
+    assert window.canvas_backend() == "pyvista"
+    assert isinstance(window._canvas, ModelCanvas)
+    assert window._diagram_renderer is not None
+
+
+@pytest.mark.gui
+def test_switch_to_plotly_preserves_selection(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.canvas_plotly import PlotlyCanvas
+
+    window = _make_window(tmp_path, qtbot)
+    window._vm.new_project()
+    window._canvas.selection.select_node(2)
+
+    window.swap_canvas_backend("plotly")
+
+    assert window.canvas_backend() == "plotly"
+    assert isinstance(window._canvas, PlotlyCanvas)
+    # One shared SelectionState: the tree/table/properties stay bound.
+    assert window._canvas.selection is window._selection
+    assert window._canvas.selection.nodes == frozenset({2})
+    # Both canvases live in the stack; the PyVista one was not destroyed.
+    assert window._canvas_stack.count() == 2
+
+
+@pytest.mark.gui
+def test_plotly_gates_pyvista_only_features(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _make_window(tmp_path, qtbot)
+    assert window._canvas.capabilities.diagrams is True
+
+    window.swap_canvas_backend("plotly")
+    caps = window._canvas.capabilities
+    assert caps.diagrams is False
+    assert caps.animation_export is False
+    assert caps.extrusions is True and caps.labels is True
+
+    window._refresh_action_enablement()
+    assert window._act_show_force_diagram.isEnabled() is False
+    assert window._act_export_th_animation.isEnabled() is False
+
+
+@pytest.mark.gui
+def test_swapping_back_reuses_the_same_canvases(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _make_window(tmp_path, qtbot)
+    pyvista_canvas = window._canvas
+
+    window.swap_canvas_backend("plotly")
+    plotly_canvas = window._canvas
+    window.swap_canvas_backend("pyvista")
+
+    assert window._canvas is pyvista_canvas
+    assert window._diagram_renderer is not None
+    # No duplicate widgets accumulate across repeated swaps.
+    window.swap_canvas_backend("plotly")
+    assert window._canvas is plotly_canvas
+    assert window._diagram_renderer is None  # diagrams are PyVista-only
+    assert window._canvas_stack.count() == 2
+
+
+@pytest.mark.gui
+def test_backend_choice_is_persisted(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _make_window(tmp_path, qtbot)
+    window.swap_canvas_backend("plotly")
+    assert window._settings.value("canvas/backend") == "plotly"
+
+    # A fresh window with the same settings boots straight into Plotly.
+    from otko.views.canvas_plotly import PlotlyCanvas
+    from otko.views.main_window import MainWindow
+
+    settings = QSettings(str(tmp_path / "otko.ini"), QSettings.Format.IniFormat)
+    second = MainWindow(settings=settings)
+    qtbot.addWidget(second)
+    assert second.canvas_backend() == "plotly"
+    assert isinstance(second._canvas, PlotlyCanvas)
+
+
+@pytest.mark.gui
+def test_working_plane_survives_the_swap(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _make_window(tmp_path, qtbot)
+    window._vm.open(_example("basic_truss"))
+    window._canvas.set_working_plane("XY", 0.0)
+
+    window.swap_canvas_backend("plotly")
+
+    assert window._canvas.working_plane_type() == "XY"
+    assert window._canvas.working_plane_offset() == 0.0
+
+
+def _example(name: str) -> str:
+    from pathlib import Path
+
+    return str(Path(__file__).resolve().parents[2] / "examples" / f"{name}.osmodel")

tests/gui/test_plot_properties.py

@@ -0,0 +1,131 @@
+"""GUI tests for Options → Plot Properties.
+
+Covers the dialog itself plus MainWindow's live-apply / persist / revert
+wiring. QSettings are injected so nothing touches real user state.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("PySide6")
+pytest.importorskip("pyvistaqt")
+
+from PySide6.QtCore import QObject, QSettings, Signal
+from PySide6.QtWidgets import QDialog
+
+
+def _settings(tmp_path):  # type: ignore[no-untyped-def]
+    return QSettings(str(tmp_path / "otko.ini"), QSettings.Format.IniFormat)
+
+
+def _window(tmp_path, qtbot):  # type: ignore[no-untyped-def]
+    from otko.views.main_window import MainWindow
+
+    window = MainWindow(settings=_settings(tmp_path))
+    qtbot.addWidget(window)
+    return window
+
+
+def test_default_style_is_the_documented_palette(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.canvas3d.style import RenderStyle
+
+    window = _window(tmp_path, qtbot)
+    assert window._style == RenderStyle()
+
+
+def test_set_plot_style_reaches_both_canvases_and_persists(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _window(tmp_path, qtbot)
+    window.swap_canvas_backend("plotly")  # build the second canvas too
+
+    window.set_plot_style(
+        window._style.with_updates(element_beam_color="#ff0000", label_font_size=20)
+    )
+
+    assert set(window._canvases) == {"pyvista", "plotly"}
+    for canvas in window._canvases.values():
+        assert canvas._style.element_beam_color == "#ff0000"
+        assert canvas._style.label_font_size == 20
+
+    # A fresh window sharing the settings file reloads the customisation.
+    from otko.views.main_window import MainWindow
+
+    fresh = MainWindow(settings=_settings(tmp_path))
+    qtbot.addWidget(fresh)
+    assert fresh._style.element_beam_color == "#ff0000"
+    assert fresh._style.label_font_size == 20
+
+
+def test_preview_does_not_persist(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    window = _window(tmp_path, qtbot)
+    window.set_plot_style(window._style.with_updates(node_color="#010203"), persist=False)
+
+    assert window._canvas._style.node_color == "#010203"
+    assert window._settings.value("plot/props") is None
+
+
+def test_cancel_reverts_the_style(tmp_path, qtbot, monkeypatch) -> None:  # type: ignore[no-untyped-def]
+    window = _window(tmp_path, qtbot)
+    original = window._style
+    preview = original.with_updates(node_color="#010203")
+
+    class _RejectingDialog(QObject):
+        changed = Signal(object)
+
+        def __init__(self, style: object, parent: QObject | None = None) -> None:
+            super().__init__(parent)
+            self._style = preview
+
+        def exec(self) -> QDialog.DialogCode:
+            self.changed.emit(preview)  # live preview happens...
+            return QDialog.DialogCode.Rejected  # ...but the user cancels
+
+        def plot_style(self):  # type: ignore[no-untyped-def]
+            return self._style
+
+    monkeypatch.setattr("otko.views.dialogs.PlotPropertiesDialog", _RejectingDialog)
+    window._on_plot_properties()
+
+    assert window._style == original
+    assert window._canvas._style.node_color == original.node_color
+    assert window._settings.value("plot/props") is None
+
+
+def test_accept_applies_and_persists(tmp_path, qtbot, monkeypatch) -> None:  # type: ignore[no-untyped-def]
+    window = _window(tmp_path, qtbot)
+    preview = window._style.with_updates(node_color="#010203")
+
+    class _AcceptingDialog(QObject):
+        changed = Signal(object)
+
+        def __init__(self, style: object, parent: QObject | None = None) -> None:
+            super().__init__(parent)
+            self._style = preview
+
+        def exec(self) -> QDialog.DialogCode:
+            return QDialog.DialogCode.Accepted
+
+        def plot_style(self):  # type: ignore[no-untyped-def]
+            return self._style
+
+    monkeypatch.setattr("otko.views.dialogs.PlotPropertiesDialog", _AcceptingDialog)
+    window._on_plot_properties()
+
+    assert window._style.node_color == "#010203"
+    assert window._canvas._style.node_color == "#010203"
+    assert "#010203" in str(window._settings.value("plot/props"))
+
+
+def test_dialog_edits_and_reset(tmp_path, qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.views.canvas3d.style import RenderStyle
+    from otko.views.dialogs import PlotPropertiesDialog
+
+    dialog = PlotPropertiesDialog(RenderStyle())
+    qtbot.addWidget(dialog)
+    assert dialog.plot_style() == RenderStyle()
+
+    dialog._colors["node_color"].set_color("#abcdef")
+    assert dialog.plot_style().node_color == "#abcdef"
+
+    dialog._reset()
+    assert dialog.plot_style() == RenderStyle()

tests/gui/test_plotly_hover.py

@@ -0,0 +1,93 @@
+"""Regression tests for the Plotly hover/snap-marker contract.
+
+``plotly_hover``/``plotly_unhover`` fire continuously while the mouse moves.
+Touching the plot on each one made it redraw per mouse move, re-firing hover
+until the stack blew (``RangeError: Maximum call stack size exceeded``) and the
+canvas froze. The marker is therefore coalesced and only touched when its
+shown/hidden state actually changes.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+pytest.importorskip("PySide6")
+pytest.importorskip("plotly")
+
+from otko.services import load_project
+
+EXAMPLES = Path(__file__).resolve().parents[2] / "examples"
+
+#: 200 mouse-move events over a *non-snap* point with snapping armed.
+_CHURN_PROBE = """(function(){
+  var gd = document.getElementById('plot');
+  var n = 0; var original = Plotly.restyle;
+  Plotly.restyle = function () { n++; return original.apply(Plotly, arguments); };
+  var point = { data: gd.data[0], customdata: [0, 0, 0] };
+  try {
+    for (var i = 0; i < 200; i++) {
+      gd.emit('plotly_hover', { points: [point] });
+      gd.emit('plotly_unhover', {});
+    }
+    return n;
+  } finally { Plotly.restyle = original; }
+})()"""
+
+_HOVER_MARKER_X = (
+    "JSON.stringify((document.getElementById('plot').data"
+    ".find(function(d){return d.meta && d.meta.kind === 'hover';}) || {}).x)"
+)
+
+_EMIT_SNAP_HOVER = """(function(){
+  var gd = document.getElementById('plot');
+  var snap = gd.data.filter(function(d){return d.meta && d.meta.kind === 'snap';})[0];
+  var point = { data: snap, customdata: [snap.x[0], snap.y[0], snap.z[0]] };
+  gd.emit('plotly_hover', { points: [point] });
+  return true;
+})()"""
+
+
+def _open_canvas(qtbot, name: str):  # type: ignore[no-untyped-def]
+    from otko.views.canvas_plotly import PlotlyCanvas
+
+    canvas = PlotlyCanvas()
+    qtbot.addWidget(canvas)
+    canvas.show_project(load_project(EXAMPLES / name))
+    qtbot.waitUntil(lambda: canvas._ready, timeout=30000)
+    qtbot.wait(1500)  # let the first Plotly.react settle
+    return canvas
+
+
+def _run_js(canvas, qtbot, script: str, timeout: int = 15000):  # type: ignore[no-untyped-def]
+    box: dict[str, object] = {}
+    canvas._web.page().runJavaScript(script, lambda value: box.update(value=value))
+    qtbot.waitUntil(lambda: "value" in box, timeout=timeout)
+    return box["value"]
+
+
+@pytest.mark.gui
+def test_ordinary_mouse_movement_does_not_redraw(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """The unfixed version restyled once per hover/unhover event."""
+    canvas = _open_canvas(qtbot, "space_frame_3d.osmodel")
+
+    restyles = _run_js(canvas, qtbot, _CHURN_PROBE)
+
+    assert restyles == 0, f"mouse movement caused {restyles} plot restyles"
+
+
+@pytest.mark.gui
+def test_snap_marker_shows_and_clears(qtbot) -> None:  # type: ignore[no-untyped-def]
+    canvas = _open_canvas(qtbot, "basic_truss.osmodel")
+    assert _run_js(canvas, qtbot, _HOVER_MARKER_X) == "[]"
+
+    canvas.set_snap_preview_enabled(True)
+    qtbot.wait(100)
+    _run_js(canvas, qtbot, _EMIT_SNAP_HOVER)
+    qtbot.wait(250)
+    assert _run_js(canvas, qtbot, _HOVER_MARKER_X) != "[]", "snap target not shown"
+
+    _run_js(canvas, qtbot, "document.getElementById('plot').emit('plotly_unhover',{})")
+    qtbot.wait(250)
+    assert _run_js(canvas, qtbot, _HOVER_MARKER_X) == "[]", "snap target not cleared"

tests/gui/test_plotly_view_preservation.py

@@ -0,0 +1,123 @@
+"""Regression tests for the Plotly canvas push contract.
+
+``Plotly.react`` resets any scene attribute the incoming layout omits, so a
+data-only push must be flagged ``preserveView`` (the JS side then carries the
+live camera and axis ranges forward). Only explicit re-frames may send the
+computed framing. These tests capture the JavaScript the canvas emits instead
+of driving the browser, so they stay fast and deterministic.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+pytest.importorskip("PySide6")
+pytest.importorskip("plotly")
+
+from otko.services import load_project
+
+EXAMPLES = Path(__file__).resolve().parents[2] / "examples"
+
+
+def _canvas_with_captured_js(qtbot):  # type: ignore[no-untyped-def]
+    from otko.views.canvas_plotly import PlotlyCanvas
+
+    canvas = PlotlyCanvas()
+    qtbot.addWidget(canvas)
+    # Pretend the page finished loading, then capture instead of running JS.
+    canvas._ready = True
+    calls: list[str] = []
+    canvas._eval = lambda js: calls.append(js)  # type: ignore[method-assign]
+    return canvas, calls
+
+
+def _parse_call(call: str) -> tuple[dict, bool]:
+    """Split a captured ``window.otkoUpdate(<json>, <flag>)`` call."""
+    body = call[len("window.otkoUpdate(") : -1]
+    literal, _sep, flag = body.rpartition(", ")
+    return json.loads(json.loads(literal)), flag == "true"
+
+
+@pytest.mark.gui
+def test_framing_push_is_not_marked_preserve_view(qtbot) -> None:  # type: ignore[no-untyped-def]
+    canvas, calls = _canvas_with_captured_js(qtbot)
+
+    canvas.show_project(load_project(EXAMPLES / "cantilever.osmodel"))
+
+    assert calls, "show_project must push a figure"
+    payload, preserve = _parse_call(calls[-1])
+    assert preserve is False, "initial push must be a re-frame"
+    # The framing push carries the camera and the padded ranges.
+    scene = payload["layout"]["scene"]
+    assert "camera" in scene
+    assert "range" in scene["xaxis"]
+    assert scene["xaxis"]["autorange"] is False
+
+
+@pytest.mark.gui
+def test_data_update_preserves_the_view(qtbot) -> None:  # type: ignore[no-untyped-def]
+    canvas, calls = _canvas_with_captured_js(qtbot)
+
+    canvas.show_project(load_project(EXAMPLES / "cantilever.osmodel"))
+    calls.clear()
+
+    canvas.selection.select_node(1)  # data-only update
+
+    assert calls, "selection change must push"
+    payload, preserve = _parse_call(calls[-1])
+    assert preserve is True, "data update must preserve the view"
+    assert "camera" not in payload["layout"]["scene"], "must not re-send the camera"
+
+
+@pytest.mark.gui
+def test_view_presets_and_reset_re_frame(qtbot) -> None:  # type: ignore[no-untyped-def]
+    canvas, calls = _canvas_with_captured_js(qtbot)
+
+    canvas.show_project(load_project(EXAMPLES / "cantilever.osmodel"))
+
+    for action in (canvas.view_xy, canvas.view_xz, canvas.view_yz, canvas.reset_camera):
+        calls.clear()
+        action()
+        assert calls[-1].endswith(", false)"), f"{action.__name__} must re-frame"
+
+
+@pytest.mark.gui
+def test_style_change_keeps_the_view_but_updates_the_layout(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """Re-colouring must not snap the camera back to the preset."""
+    from otko.views.canvas3d.style import RenderStyle
+
+    canvas, calls = _canvas_with_captured_js(qtbot)
+    canvas.show_project(load_project(EXAMPLES / "cantilever.osmodel"))
+    calls.clear()
+
+    canvas.set_style(RenderStyle(show_axis_outline=True))
+
+    assert calls, "a style change must push"
+    payload, preserve = _parse_call(calls[-1])
+    assert preserve is True, "colour changes must not re-frame the view"
+    assert payload["layout"]["scene"]["xaxis"]["showgrid"] is True  # layout applied
+
+
+@pytest.mark.gui
+def test_js_console_messages_reach_the_log(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """A plotly failure inside the page must not be invisible from Python."""
+    from PySide6.QtWebEngineCore import QWebEnginePage
+
+    from otko.views.canvas_plotly.plotly_canvas import _LoggingPage
+
+    messages: list[tuple[str, str]] = []
+    page = _LoggingPage(lambda msg, severity="info": messages.append((severity, msg)))
+
+    levels = QWebEnginePage.JavaScriptConsoleMessageLevel
+    page.javaScriptConsoleMessage(levels.ErrorMessageLevel, "boom", 7, "file:///tmp/plot.html")
+    page.javaScriptConsoleMessage(levels.WarningMessageLevel, "careful", 8, "file:///tmp/plot.html")
+    page.javaScriptConsoleMessage(levels.InfoMessageLevel, "hello", 9, "")
+
+    assert messages == [
+        ("error", "[web] plot.html:7 boom"),
+        ("warning", "[web] plot.html:8 careful"),
+        ("info", "[web] plotly:9 hello"),
+    ]

tests/unit/test_architecture.py

@@ -2,11 +2,12 @@
 
 The documented dependency stack is ``views -> viewmodels -> services -> core``;
 each layer may only import from itself or a lower layer, ``core`` stays free of
-Qt/solver/plotting libraries, and the rendering backend is confined to
-``views/canvas3d``. These tests never import the application modules; they read
-the source tree with ``ast`` so the import discipline can be checked without
-executing Qt or the solver. Directories that do not exist yet simply have
-nothing to walk, which trivially satisfies a "forbidden import" rule.
+Qt/solver/plotting libraries, and the rendering backends are confined to the
+canvas packages (``views/canvas3d``, ``views/canvas_plotly``). These tests never
+import the application modules; they read the source tree with ``ast`` so the
+import discipline can be checked without executing Qt or the solver.
+Directories that do not exist yet simply have nothing to walk, which trivially
+satisfies a "forbidden import" rule.
 """
 
 from __future__ import annotations
@@ -16,7 +17,10 @@ from pathlib import Path
 
 ROOT = Path(__file__).resolve().parents[2]
 SRC = ROOT / "src" / "otko"
-CANVAS = SRC / "views" / "canvas3d"
+CANVAS_DIRS = (
+    SRC / "views" / "canvas3d",
+    SRC / "views" / "canvas_plotly",
+)
 
 # Qt binding roots, in every spelling the project might use.
 QT_ROOTS = {"PySide6", "PyQt5", "PyQt6", "PySide"}
@@ -32,7 +36,7 @@ CORE_FORBIDDEN = QT_ROOTS | {
     "matplotlib",
 }
 
-# Only views/canvas3d/ may touch a 3D or plotting backend.
+# Only the canvas packages may touch a 3D or plotting backend.
 VTK_ROOTS = {"pyvista", "pyvistaqt", "vtk", "plotly"}
 
 
@@ -75,14 +79,14 @@ def _violations(
     forbidden: set[str],
     *,
     skip: tuple[Path, ...] = (),
-    exclude_dir: Path | None = None,
+    exclude_dirs: tuple[Path, ...] = (),
 ) -> list[str]:
     """Return ``"<path>: <module>"`` entries for every forbidden import found."""
     found: list[str] = []
     for path in _iter_python_files(root):
         if path in skip:
             continue
-        if exclude_dir is not None and _is_within(path, exclude_dir):
+        if any(_is_within(path, directory) for directory in exclude_dirs):
             continue
         for module in sorted(_root_imports(path) & forbidden):
             found.append(f"{path.relative_to(ROOT)}: {module}")
@@ -112,9 +116,9 @@ def test_architecture_views_no_solver() -> None:
 
 
 def test_no_vtk_import_outside_canvas() -> None:
-    """3D/plot backends stay under views/canvas3d/."""
-    violations = _violations(SRC, VTK_ROOTS, exclude_dir=CANVAS)
-    assert violations == [], f"backend imports outside views/canvas3d/: {violations}"
+    """3D/plot backends stay under the canvas packages."""
+    violations = _violations(SRC, VTK_ROOTS, exclude_dirs=CANVAS_DIRS)
+    assert violations == [], f"backend imports outside the canvas packages: {violations}"
 
 
 # The documented dependency stack; ``commands`` sits beside viewmodels (a VM

tests/unit/test_plotly_trace_builder.py

@@ -0,0 +1,262 @@
+"""Unit tests for the pure Plotly trace builder (no Qt, no plotly import)."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import numpy as np
+import pytest
+
+from otko.services import load_project
+from otko.views.canvas3d.style import RenderStyle
+from otko.views.canvas_plotly.trace_builder import (
+    PlotlyTraceBuilder,
+    SceneOptions,
+)
+
+EXAMPLES = Path(__file__).resolve().parents[2] / "examples"
+
+
+def _load(name: str):  # type: ignore[no-untyped-def]
+    return load_project(EXAMPLES / f"{name}.osmodel")
+
+
+def _traces(scene, name: str) -> list[dict]:  # type: ignore[no-untyped-def]
+    return [trace for trace in scene.data if trace.get("name") == name]
+
+
+def _kinds(scene) -> list[str]:  # type: ignore[no-untyped-def]
+    return [trace.get("meta", {}).get("kind", trace["type"]) for trace in scene.data]
+
+
+def test_builds_grid_nodes_and_frames() -> None:
+    scene = PlotlyTraceBuilder().build(_load("basic_truss"), SceneOptions())
+    names = {trace.get("name") for trace in scene.data}
+    assert "grid-active" in names
+    assert "nodes" in names
+    assert "elements" in names
+    assert scene.diagonal > 0
+    # The hover-snap marker is always present so JS can restyle it.
+    assert scene.data[scene.hover_trace]["meta"]["kind"] == "hover"
+
+
+def test_nodes_carry_ids_as_customdata() -> None:
+    project = _load("basic_truss")
+    scene = PlotlyTraceBuilder().build(project, SceneOptions())
+    (nodes,) = _traces(scene, "nodes")
+    assert nodes["meta"]["kind"] == "node"
+    assert nodes["customdata"] == [node.id for node in project.nodes]
+    assert len(nodes["marker"]["color"]) == len(project.nodes)
+
+
+def _color_for_index(trace: dict, index: float) -> str:
+    """Map a ``line.color`` scalar index to its colour via the colorscale."""
+    scale = sorted(trace["line"]["colorscale"], key=lambda item: item[0])
+    return scale[int(index)][1]
+
+
+def test_frames_are_one_trace_with_per_element_colours() -> None:
+    """A single line trace carries the palette through its ``line.color`` array."""
+    project = _load("cantilever")
+    style = RenderStyle()
+    scene = PlotlyTraceBuilder(style).build(
+        project, SceneOptions(selection_elements=frozenset({2}))
+    )
+    frames = _traces(scene, "elements")
+    assert len(frames) == 1, "selection must not split the frame trace"
+    line = frames[0]["line"]
+    assert isinstance(line["color"], list)
+    assert len(line["color"]) == len(frames[0]["x"])
+    assert line["colorscale"]
+
+    colors = line["color"]
+    customdata = frames[0]["customdata"]
+
+    def index_of(element_id: int) -> set[float]:
+        return {colors[k] for k, value in enumerate(customdata) if value == element_id}
+
+    selected = index_of(2)
+    unselected = index_of(1)
+    assert len(selected) == 1 and len(unselected) == 1
+    assert selected != unselected
+    assert _color_for_index(frames[0], next(iter(selected))) == style.selected_color
+    assert _color_for_index(frames[0], next(iter(unselected))) == style.element_beam_color
+
+
+def test_element_palette_distinguishes_beam_from_truss() -> None:
+    style = RenderStyle()
+    builder = PlotlyTraceBuilder(style)
+    beam = _traces(builder.build(_load("cantilever"), SceneOptions()), "elements")[0]
+    truss = _traces(builder.build(_load("basic_truss"), SceneOptions()), "elements")[0]
+    assert _color_for_index(beam, beam["line"]["color"][0]) == style.element_beam_color
+    assert _color_for_index(truss, truss["line"]["color"][0]) == style.element_truss_color
+
+
+def test_selected_node_is_highlighted_by_colour() -> None:
+    project = _load("basic_truss")
+    target = project.nodes[1].id
+    scene = PlotlyTraceBuilder().build(project, SceneOptions(selection_nodes=frozenset({target})))
+    (nodes,) = _traces(scene, "nodes")
+    ids = nodes["customdata"]
+    assert nodes["marker"]["color"][ids.index(target)] != nodes["marker"]["color"][0]
+
+
+def test_working_plane_filters_grid_segments() -> None:
+    project = _load("basic_truss")
+    builder = PlotlyTraceBuilder()
+    iso = builder.build(project, SceneOptions())
+    plane = builder.build(project, SceneOptions(working_plane=("XY", 0.0)))
+    (iso_grid,) = _traces(iso, "grid-active")
+    (plane_grid,) = _traces(plane, "grid-active")
+    assert len(plane_grid["x"]) <= len(iso_grid["x"])
+
+
+def test_snap_targets_carry_world_coordinates() -> None:
+    scene = PlotlyTraceBuilder().build(_load("basic_truss"), SceneOptions())
+    (snap,) = _traces(scene, "snap")
+    assert snap["meta"]["kind"] == "snap"
+    assert snap["customdata"], "expected at least one snappable intersection"
+    assert all(len(point) == 3 for point in snap["customdata"])
+    assert all(isinstance(coord, float) for point in snap["customdata"] for coord in point)
+
+
+def test_extrusions_add_a_mesh3d_trace() -> None:
+    project = _load("cantilever")
+    scene = PlotlyTraceBuilder().build(project, SceneOptions(show_extrusions=True))
+    (mesh,) = _traces(scene, "extrusions")
+    assert mesh["type"] == "mesh3d"
+    assert mesh["i"] and mesh["j"] and mesh["k"]
+    assert max(mesh["i"]) < len(mesh["x"])
+
+
+def test_local_axes_add_coloured_cones() -> None:
+    project = _load("cantilever")
+    scene = PlotlyTraceBuilder().build(project, SceneOptions(show_local_axes=True))
+    names = {trace.get("name") for trace in scene.data}
+    assert {"local-x", "local-y", "local-z"} <= names
+    for trace in scene.data:
+        if trace.get("name", "").startswith("local-"):
+            assert trace["type"] == "cone"
+            assert len(trace["u"]) == len(trace["x"]) > 0
+
+
+def test_labels_traces_are_text_only() -> None:
+    project = _load("cantilever")
+    scene = PlotlyTraceBuilder().build(
+        project, SceneOptions(show_node_labels=True, show_element_labels=True)
+    )
+    (node_labels,) = _traces(scene, "node-labels")
+    (element_labels,) = _traces(scene, "element-labels")
+    assert node_labels["mode"] == "text"
+    assert len(node_labels["text"]) == len(project.nodes)
+    assert element_labels["mode"] == "text"
+
+
+def test_deformation_shifts_node_coordinates() -> None:
+    project = _load("cantilever")
+
+    class _Shift:
+        def shifted(self, points: np.ndarray, node_ids: list[int]) -> np.ndarray:
+            out = points.copy()
+            out[:, 2] += 1.0
+            return out
+
+    base = PlotlyTraceBuilder().build(project, SceneOptions())
+    moved = PlotlyTraceBuilder().build(project, SceneOptions(deformation=_Shift()))
+    (base_nodes,) = _traces(base, "nodes")
+    (moved_nodes,) = _traces(moved, "nodes")
+    assert moved_nodes["z"] == pytest.approx([z + 1.0 for z in base_nodes["z"]])
+
+
+def test_payload_is_json_serialisable() -> None:
+    scene = PlotlyTraceBuilder().build(
+        _load("cantilever"),
+        SceneOptions(
+            selection_nodes=frozenset({1}),
+            selection_elements=frozenset({1}),
+            working_plane=("XY", 0.0),
+            show_extrusions=True,
+            show_local_axes=True,
+            show_node_labels=True,
+            show_element_labels=True,
+        ),
+    )
+    payload = json.dumps(scene.to_payload())
+    assert '"data"' in payload and '"layout"' in payload
+    assert scene.layout["scene"]["aspectmode"] == "data"
+
+
+def test_empty_project_yields_only_the_hover_marker() -> None:
+    scene = PlotlyTraceBuilder().build(None, SceneOptions())
+    assert scene.data == []
+    assert scene.hover_trace == -1
+
+    from otko.core import Project
+
+    empty = PlotlyTraceBuilder().build(Project(ndm=3, ndf=6), SceneOptions())
+    assert _kinds(empty) == ["hover"]
+
+
+def test_frame_trace_meta_marks_elements_pickable() -> None:
+    project = _load("cantilever")
+    scene = PlotlyTraceBuilder().build(project, SceneOptions())
+    (frames,) = _traces(scene, "elements")
+    assert frames["meta"]["kind"] == "element"
+    # None separators break the line into per-element segments.
+    assert None in frames["x"]
+    assert len(frames["customdata"]) == len(frames["x"])
+
+
+# ── scene recipe (opstool-derived framing) ───────────────────────────────
+def test_axis_bounds_are_padded_around_the_model() -> None:
+    project = _load("cantilever")
+    scene = PlotlyTraceBuilder().build(project, SceneOptions())
+    points = np.array([node.coords for node in project.nodes], dtype=float)
+    for index, axis in enumerate(("x", "y", "z")):
+        low, high = scene.axis_bounds[axis]
+        assert low < points[:, index].min() or low <= points[:, index].min()
+        assert high > points[:, index].max() or high >= points[:, index].max()
+    assert scene.diagonal > 0
+
+
+def test_planar_model_gets_unit_slack_on_the_flat_axis() -> None:
+    project = _load("basic_truss")
+    scene = PlotlyTraceBuilder().build(project, SceneOptions())
+    low, high = scene.axis_bounds["z"]
+    assert high - low > 0  # a degenerate axis must not collapse the view
+
+
+def test_axis_overrides_pin_ranges_with_autorange_off() -> None:
+    scene = PlotlyTraceBuilder().build(_load("cantilever"), SceneOptions())
+    overrides = scene.axis_overrides()
+    assert set(overrides) == {"xaxis", "yaxis", "zaxis"}
+    for axis, override in overrides.items():
+        assert override["autorange"] is False
+        assert len(override["range"]) == 2
+        assert override["range"] == list(scene.axis_bounds[axis[0]])
+
+
+def test_hover_templates_identify_entities() -> None:
+    scene = PlotlyTraceBuilder().build(_load("basic_truss"), SceneOptions())
+    (nodes,) = _traces(scene, "nodes")
+    (frames,) = _traces(scene, "elements")
+    assert nodes["hovertemplate"] == "Node #%{customdata}<extra></extra>"
+    assert frames["hovertemplate"] == "Element #%{customdata}<extra></extra>"
+    # Hover must not fall back to the raw-id "skip" mode.
+    assert "hoverinfo" not in nodes
+
+
+def test_axis_outline_flag_toggles_grid_and_ticks() -> None:
+    plain = RenderStyle()
+    outlined = RenderStyle(show_axis_outline=True)
+    project = _load("cantilever")
+    scene_plain = PlotlyTraceBuilder(plain).build(project, SceneOptions())
+    scene_outlined = PlotlyTraceBuilder(outlined).build(project, SceneOptions())
+
+    off_axis = scene_plain.layout["scene"]["xaxis"]
+    on_axis = scene_outlined.layout["scene"]["xaxis"]
+    assert off_axis["showgrid"] is False and off_axis["showticklabels"] is False
+    assert on_axis["showgrid"] is True and on_axis["showticklabels"] is True
+    # The coloured axis lines stay visible either way (orientation cue).
+    assert off_axis["visible"] is True and on_axis["visible"] is True

tests/unit/test_render_style.py

@@ -0,0 +1,117 @@
+"""Unit tests for the shared render style and its editable subset."""
+
+from __future__ import annotations
+
+import dataclasses
+
+import pytest
+
+from otko.core import ElasticBeamColumn, Node, Project, TrussElement, ZeroLengthElement
+from otko.views.canvas3d.style import (
+    EDITABLE_FIELDS,
+    SELECTED_STATE,
+    STYLE_FIELDS,
+    RenderStyle,
+    element_family_index,
+    family_palette,
+)
+
+
+def test_defaults_are_the_documented_palette() -> None:
+    style = RenderStyle()
+    assert style.node_color == "#d9d9d9"
+    assert style.selected_color == "#00ffff"
+    # opstool-derived element palette.
+    assert style.element_beam_color == "#0652ff"
+    assert style.element_truss_color == "#FF8C00"
+    assert style.element_link_color == "#39FF14"
+    assert len(style.response_scale_colors) == 10
+    assert style.response_scale_colors[0] == "#313695"  # cold end
+    assert style.response_scale_colors[-1] == "#a50026"  # warm end
+
+
+def test_with_updates_is_immutable() -> None:
+    original = RenderStyle()
+    updated = original.with_updates(node_color="#123456", label_font_size=20)
+    assert updated is not original
+    assert updated.node_color == "#123456"
+    assert updated.label_font_size == 20
+    # The source instance is untouched (frozen dataclass contract).
+    assert original.node_color == "#d9d9d9"
+    assert original.label_font_size == 12
+
+
+def test_editable_subset_matches_the_field_table() -> None:
+    style = RenderStyle()
+    assert RenderStyle.EDITABLE == EDITABLE_FIELDS
+    assert set(style.editable_values()) == set(EDITABLE_FIELDS)
+    # Every advertised field must exist on the dataclass with the right kind.
+    kinds = {name: kind for name, _label, kind in STYLE_FIELDS}
+    for name in EDITABLE_FIELDS:
+        assert hasattr(style, name), name
+        assert kinds[name] in {"color", "float", "int", "bool"}
+
+
+def test_response_colorscale_is_an_evenly_spaced_mapping() -> None:
+    style = RenderStyle()
+    scale = style.response_colorscale()
+    assert len(scale) == len(style.response_scale_colors)
+    assert scale[0] == (0.0, style.response_scale_colors[0])
+    assert scale[-1] == (1.0, style.response_scale_colors[-1])
+    positions = [position for position, _color in scale]
+    assert positions == sorted(positions)
+
+
+@pytest.mark.parametrize(
+    ("element", "expected"),
+    [
+        (ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1), 0),
+        (TrussElement(id=2, nodes=(1, 2), area=0.01, material_id=1), 1),
+        (ZeroLengthElement(id=3, nodes=(1, 1), material_ids=(1,), dofs=(1,)), 2),
+    ],
+)
+def test_element_family_index(element: object, expected: int) -> None:
+    assert element_family_index(element) == expected
+
+
+def test_family_palette_order_and_selected_slot() -> None:
+    style = RenderStyle(node_color="#010101")
+    palette = family_palette(style)
+    assert palette == [
+        style.element_beam_color,
+        style.element_truss_color,
+        style.element_link_color,
+        style.selected_color,
+    ]
+    assert len(palette) - 1 == SELECTED_STATE
+
+
+def test_editable_values_round_trip_through_replace() -> None:
+    style = RenderStyle()
+    values = style.editable_values()
+    assert RenderStyle(**values) == style
+    # And the persisted shape is JSON-friendly (str/int/float only).
+    import json
+
+    assert json.loads(json.dumps(values)) == values
+
+
+def test_style_is_a_frozen_slots_dataclass() -> None:
+    assert dataclasses.is_dataclass(RenderStyle)
+    with pytest.raises(dataclasses.FrozenInstanceError):
+        RenderStyle().node_color = "#000000"  # type: ignore[misc]
+
+
+def test_project_builder_helper_is_unused_but_valid() -> None:
+    """A style constructed from a project's elements classifies every family."""
+    project = Project(
+        ndm=2,
+        ndf=3,
+        nodes=[Node(id=1, coords=(0, 0, 0)), Node(id=2, coords=(1, 0, 0))],
+        elements=[
+            ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1),
+            TrussElement(id=2, nodes=(1, 2), area=0.01, material_id=1),
+        ],
+    )
+    indices = [element_family_index(el) for el in project.elements]
+    assert indices == [0, 1]

tests/unit/test_renderer.py

@@ -24,6 +24,7 @@ from otko.views.canvas3d.model_renderer import (
     RendererMode,
     _classify_support,
 )
+from otko.views.canvas3d.style import SELECTED_STATE
 
 
 # ──────────────────────────── support classification ────────────────────────────
@@ -125,9 +126,13 @@ def test_update_selection_writes_state_array(offscreen_plotter, small_3d_project
     frame_states = np.asarray(r._frame_pd.cell_data["_oss_state"]).tolist()
     # Nodes 1 and 3 selected → row 0 and row 2
     assert node_states == [1, 0, 1]
-    # Element 2 selected → it's the second frame (index 1 in frame_ids_ordered)
+    # Frame cells carry the palette slot: the element family normally, and
+    # SELECTED_STATE when selected.
     selected_frame_idx = r._frame_id_to_row[2]
-    assert frame_states[selected_frame_idx] == 1
+    assert frame_states[selected_frame_idx] == SELECTED_STATE
+    for index, family in enumerate(r._frame_family):
+        expected = SELECTED_STATE if index == selected_frame_idx else family
+        assert frame_states[index] == expected
 
 
 def test_clear_selection(offscreen_plotter, small_3d_project) -> None:  # type: ignore[no-untyped-def]