Fix scale parameter for polygons with datashader (#473)

The datashader rendering path only applied the `scale` parameter to
circle geometries (Points with radius). Polygon and MultiPolygon
geometries were passed through unscaled. This adds scaling via
`shapely.affinity.scale()` around each geometry's centroid, matching
the existing matplotlib path behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/render.py

@@ -394,6 +394,15 @@ def _render_shapes(
             scale = radius_numeric * render_params.scale
             shapes.loc[is_point, "geometry"] = _geometry[is_point].buffer(scale.to_numpy())
 
+        # Handle polygon/multipolygon scaling
+        is_polygon = _geometry.type.isin(["Polygon", "MultiPolygon"])
+        if is_polygon.any() and render_params.scale != 1.0:
+            from shapely import affinity
+
+            shapes.loc[is_polygon, "geometry"] = _geometry[is_polygon].apply(
+                lambda geom: affinity.scale(geom, xfact=render_params.scale, yfact=render_params.scale)
+            )
+
         # apply transformations to the individual points
         tm = trans.get_matrix()
         transformed_geometry = shapes["geometry"].transform(

tests/pl/test_render_shapes.py

@@ -242,6 +242,10 @@ def test_plot_can_color_from_geodataframe(self, sdata_blobs: SpatialData):
     def test_plot_can_scale_shapes(self, sdata_blobs: SpatialData):
         sdata_blobs.pl.render_shapes(element="blobs_circles", scale=0.5).pl.show()
 
+    def test_plot_can_scale_polygons_datashader(self, sdata_blobs: SpatialData):
+        # Regression test for #473: scale parameter had no effect on polygons with datashader
+        sdata_blobs.pl.render_shapes(element="blobs_polygons", method="datashader", scale=2.0).pl.show()
+
     def test_plot_can_filter_with_groups(self, sdata_blobs: SpatialData):
         _, axs = plt.subplots(nrows=1, ncols=2, layout="tight")