build: add plotly to the GUI extra

Supplies the offline plotly.js bundle for the new canvas backend. The
GUI CI job now installs the gui extra so the Qt/WebEngine tests run
instead of being import-skipped.

.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:

pyproject.toml

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