Fix datashader elements invisible with non-square aspect ratios (#544)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/utils.py

@@ -2895,13 +2895,16 @@ def _get_extent_and_range_for_datashader_canvas(
             ]
         )
 
-    # compute canvas size in pixels close to the actual image size to speed up computation
+    # Compute canvas size in pixels, capped at the figure's display resolution.
+    # Using np.max ensures the canvas never exceeds display pixels on either axis,
+    # preventing pixel-based operations (spread, line_width) from being downscaled
+    # to sub-pixel size when the data aspect ratio differs from the figure's.
     plot_width = x_ext[1] - x_ext[0]
     plot_height = y_ext[1] - y_ext[0]
     plot_width_px = int(round(fig_params.fig.get_size_inches()[0] * fig_params.fig.dpi))
     plot_height_px = int(round(fig_params.fig.get_size_inches()[1] * fig_params.fig.dpi))
     factor: float
-    factor = np.min([plot_width / plot_width_px, plot_height / plot_height_px])
+    factor = np.max([plot_width / plot_width_px, plot_height / plot_height_px])
     plot_width = int(np.round(plot_width / factor))
     plot_height = int(np.round(plot_height / factor))
 

tests/_images/Points_datashader_can_transform_points.png

@@ -606,3 +606,14 @@ def test_plot_datashader_single_category_points(sdata_blobs: SpatialData):
         method="datashader",
         size=5,
     ).pl.show()
+
+
+def test_datashader_points_visible_with_nonuniform_scale(sdata_blobs: SpatialData):
+    """Datashader points must remain visible when data has a non-square aspect ratio.
+
+    Regression test for https://github.com/scverse/spatialdata-plot/issues/445.
+    Before the fix, the datashader canvas was oversized on the longer axis, causing
+    spread(px) to be downscaled to sub-pixel size on display.
+    """
+    _set_transformations(sdata_blobs["blobs_points"], {"global": Scale([1, 5], axes=("x", "y"))})
+    sdata_blobs.pl.render_points("blobs_points", method="datashader", color="black").pl.show()