Fix datashader path ignoring na_color transparency for continuous data (#565)

When na_color is fully transparent (alpha="00"), skip the NaN overlay
entirely in the datashader continuous path for both shapes and points.
Previously, _hex_no_alpha stripped the alpha channel before passing to
ds.tf.shade, turning transparent "#d3d3d300" into opaque "#d3d3d3".

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

Author: timtreis

src/spatialdata_plot/pl/render.py

@@ -530,6 +530,9 @@ def _render_shapes(
 
         agg, color_span = _apply_ds_norm(agg, norm)
         na_color_hex = _hex_no_alpha(render_params.cmap_params.na_color.get_hex())
+        # Skip NaN overlay when na_color is fully transparent (#565)
+        if render_params.cmap_params.na_color.alpha == "00":
+            nan_agg = None
         color_key = _build_color_key(
             transformed_element,
             col_for_color,
@@ -925,6 +928,9 @@ def _render_points(
 
         agg, color_span = _apply_ds_norm(agg, norm)
         na_color_hex = _hex_no_alpha(render_params.cmap_params.na_color.get_hex())
+        # Skip NaN overlay when na_color is fully transparent (#565)
+        if render_params.cmap_params.na_color.alpha == "00":
+            nan_agg = None
         color_key = _build_color_key(
             transformed_element,
             col_for_color,

tests/pl/test_render_points.py

@@ -913,3 +913,25 @@ def test_shade_categorical_cmap_used_when_no_color_key():
     shaded_blue = _ds_shade_categorical(agg, None, np.array(["#0000ff"] * 100), alpha=1.0)
     # Different color_vector[0] values should produce different shaded output
     assert not np.array_equal(np.asarray(shaded_red), np.asarray(shaded_blue))
+
+
+def test_datashader_na_color_none_no_nan_overlay_points(sdata_blobs: SpatialData):
+    """Datashader must not render NaN overlay when na_color is fully transparent.
+
+    Regression test for https://github.com/scverse/spatialdata-plot/issues/565.
+    """
+    pts = sdata_blobs.points["blobs_points"].compute()
+    n = len(pts)
+    values = np.full(n, np.nan)
+    values[: n // 2] = np.random.default_rng(0).uniform(0, 100, n // 2)
+    pts["val"] = values
+    sdata_blobs.points["blobs_points"] = PointsModel.parse(pts)
+
+    fig, ax = plt.subplots()
+    sdata_blobs.pl.render_points("blobs_points", color="val", na_color=None, method="datashader").pl.show(ax=ax)
+
+    assert len(ax.get_images()) == 1, (
+        f"Expected 1 image (no NaN overlay), got {len(ax.get_images())}; "
+        "datashader is still rendering an opaque NaN overlay despite na_color=None"
+    )
+    plt.close(fig)

tests/pl/test_render_shapes.py

@@ -1210,3 +1210,45 @@ def test_datashader_colorbar_range_matches_data(sdata_blobs: SpatialData):
     )
     assert cbar_vmin >= data_min * 0.99 - 0.01, f"Colorbar min ({cbar_vmin:.2f}) is below data min ({data_min:.2f})"
     plt.close(fig)
+
+
+def test_datashader_na_color_none_no_nan_overlay(sdata_blobs: SpatialData):
+    """Datashader must not render NaN overlay when na_color is fully transparent.
+
+    Regression test for https://github.com/scverse/spatialdata-plot/issues/565.
+    Before the fix, _hex_no_alpha stripped the transparency from na_color=None,
+    causing NaN shapes to render as opaque grey.
+    """
+    n = len(sdata_blobs.shapes["blobs_circles"])
+    values = np.full(n, np.nan)
+    values[: n // 2] = np.random.default_rng(0).uniform(0, 100, n // 2)
+    sdata_blobs.shapes["blobs_circles"]["val"] = values
+
+    fig, ax = plt.subplots()
+    sdata_blobs.pl.render_shapes("blobs_circles", color="val", na_color=None, method="datashader").pl.show(ax=ax)
+
+    # With na_color=None the NaN overlay should be skipped: only 1 image (the main data).
+    # Before the fix there were 2 images (NaN overlay + main data).
+    assert len(ax.get_images()) == 1, (
+        f"Expected 1 image (no NaN overlay), got {len(ax.get_images())}; "
+        "datashader is still rendering an opaque NaN overlay despite na_color=None"
+    )
+    plt.close(fig)
+
+
+def test_datashader_na_color_opaque_renders_nan_overlay(sdata_blobs: SpatialData):
+    """Datashader must still render the NaN overlay when na_color is opaque."""
+    n = len(sdata_blobs.shapes["blobs_circles"])
+    values = np.full(n, np.nan)
+    values[: n // 2] = np.random.default_rng(0).uniform(0, 100, n // 2)
+    sdata_blobs.shapes["blobs_circles"]["val"] = values
+
+    fig, ax = plt.subplots()
+    sdata_blobs.pl.render_shapes("blobs_circles", color="val", na_color="red", method="datashader").pl.show(ax=ax)
+
+    # With opaque na_color, the NaN overlay should be present: 2 images.
+    assert len(ax.get_images()) == 2, (
+        f"Expected 2 images (NaN overlay + main data), got {len(ax.get_images())}; "
+        "NaN overlay is missing for opaque na_color"
+    )
+    plt.close(fig)