Allow per-channel norms without explicit cmap

When norm is a list but no cmap is provided, auto-generate a default
cmap list (one per norm) so the per-channel rendering path works.
Users shouldn't need to write cmap=[plt.cm.gray]*3 just to use
per-channel normalization.

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

Author: timtreis

src/spatialdata_plot/pl/basic.py

@@ -548,7 +548,7 @@ def render_images(
             Colormap normalization for continuous annotations, see :class:`matplotlib.colors.Normalize`.
             A single :class:`~matplotlib.colors.Normalize` applies to all channels.
             A list of :class:`~matplotlib.colors.Normalize` objects applies per-channel
-            (length must match the number of colormaps/channels).
+            (length must match the number of channels).
         na_color : ColorLike | None, default "default" (gets set to "lightgray")
             Color to be used for NAs values, if present. Can either be a named color ("red"), a hex representation
             ("#000000ff") or a list of floats that represent RGB/RGBA values (1.0, 0.0, 0.0, 1.0). When None, the values
@@ -632,8 +632,14 @@ def render_images(
 
         for element, param_values in params_dict.items():
             cmap_params: list[CmapParams] | CmapParams
-            # Resolve which cmap to use for norm-list path vs scalar path.
+            # Resolve which cmap to use for the norm-list path vs scalar path.
             effective_cmap = param_values.get("cmap") if isinstance(norm, list) else cmap
+
+            # When the user passes per-channel norms without explicit cmaps,
+            # generate a default cmap list so the per-channel path works.
+            if isinstance(norm, list) and len(norm) > 1 and not isinstance(effective_cmap, list):
+                effective_cmap = [None] * len(norm)
+
             if isinstance(effective_cmap, list) and len(effective_cmap) > 1:
                 if isinstance(norm, list):
                     if len(norm) != len(effective_cmap):
@@ -654,16 +660,7 @@ def render_images(
                 ]
 
             else:
-                if isinstance(norm, list):
-                    if len(norm) == 1:
-                        norm_scalar: Normalize | None = norm[0]
-                    else:
-                        raise ValueError(
-                            "When 'norm' is a list, you must also pass a list of colormaps via 'cmap' "
-                            "with matching length, or use a single Normalize."
-                        )
-                else:
-                    norm_scalar = norm
+                norm_scalar = norm[0] if isinstance(norm, list) else norm
                 scalar_cmap = effective_cmap[0] if isinstance(effective_cmap, list) else cmap
                 cmap_params = _prepare_cmap_norm(
                     cmap=scalar_cmap,

tests/pl/test_render_images.py

@@ -475,11 +475,13 @@ def test_norm_list_with_invalid_element_raises():
         sdata.pl.render_images("img", norm=["not_a_norm"]).pl.show()
 
 
-def test_norm_list_without_cmap_list_raises():
-    """Norm list requires explicit cmap list."""
+def test_norm_list_without_explicit_cmap():
+    """Per-channel norms work without explicit cmap (auto-assigns default cmap per channel)."""
     sdata = _make_multichannel_sdata()
-    with pytest.raises(ValueError, match="must also pass a list of colormaps"):
-        sdata.pl.render_images("img", norm=[Normalize(0, 1)] * 3).pl.show()
+    norms = [Normalize(0, 0.05), Normalize(0, 1.0), Normalize(0, 0.5)]
+    fig, ax = plt.subplots()
+    sdata.pl.render_images("img", channel=[0, 1, 2], norm=norms).pl.show(ax=ax)
+    plt.close(fig)
 
 
 def test_cmap_matches_selected_channels_not_full_image(sdata_blobs: SpatialData):