Fix flat hue in categorical density: drop min_alpha floor

For categorical density the only signal available to encode count is
per-pixel alpha (hue is fixed per category by the color key). The
existing min_alpha of ~254 (set so scatter plots render every point at
full opacity) clamps that signal to a single value, so all three
gene clusters were rendering as flat, uniform blobs.

Under density=True the categorical shade path now uses min_alpha=40
(~15%): low enough to give the count-driven alpha real range, high
enough to keep sparse-edge pixels visible under density_how="linear".
Plain density (continuous shade path) is unaffected -- its viridis
gradient already encodes count via hue.

Author: timtreis

src/spatialdata_plot/pl/_datashader.py

@@ -281,6 +281,7 @@ def _ds_shade_categorical(
     alpha: float,
     spread_px: int | None = None,
     how: str = "linear",
+    density: bool = False,
 ) -> Any:
     """Shade a categorical or no-color datashader aggregate."""
     ds_cmap = None
@@ -289,12 +290,19 @@ def _ds_shade_categorical(
         if isinstance(ds_cmap, str) and ds_cmap[0] == "#":
             ds_cmap = _hex_no_alpha(ds_cmap)
 
+    # The default min_alpha (~254) is a near-full-opacity floor — right for scatter
+    # plots, but it collapses the count-driven alpha range and makes categorical
+    # density read as a flat hue cloud. Drop the floor under density so per-pixel
+    # alpha can actually encode count. A small non-zero floor (~15%) keeps the
+    # sparse edges visible under density_how="linear" instead of vanishing.
+    min_alpha = 40.0 if density else _convert_alpha_to_datashader_range(alpha)
+
     agg_to_shade = ds.tf.spread(agg, px=spread_px) if spread_px is not None else agg
     shaded = _datashader_map_aggregate_to_color(
         agg_to_shade,
         cmap=ds_cmap,
         color_key=color_key,
-        min_alpha=_convert_alpha_to_datashader_range(alpha),
+        min_alpha=min_alpha,
         how=how,
     )
     return _apply_user_alpha(shaded, alpha)

src/spatialdata_plot/pl/render.py

@@ -1105,6 +1105,7 @@ def _render_points(
                 render_params.alpha,
                 spread_px=px,
                 how=shade_how,
+                density=render_params.density,
             )
         else:
             shaded, nan_shaded, reduction_bounds = _ds_shade_continuous(