fix: continuous colorscale strings with discrete-only chart types

Passing a continuous colorscale like `colors='turbo'` to area(), line(),
fast_bar(), box(), or pie() previously failed because `resolve_colors`
set `color_continuous_scale` which these px functions don't accept. Now
samples from the continuous scale into a discrete sequence for these
chart types. bar() and scatter() natively support continuous scales so
are left unchanged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: FBumann

xarray_plotly/common.py

@@ -251,7 +251,18 @@ def _get_qualitative_scale_names() -> frozenset[str]:
     )
 
 
-def resolve_colors(colors: Colors, px_kwargs: dict[str, Any]) -> dict[str, Any]:
+def _sample_colorscale(name: str, n: int = 10) -> list[str]:
+    """Sample *n* evenly-spaced colors from a named Plotly colorscale."""
+    scale = px.colors.get_colorscale(name)
+    return px.colors.sample_colorscale(scale, [i / (n - 1) for i in range(n)])
+
+
+def resolve_colors(
+    colors: Colors,
+    px_kwargs: dict[str, Any],
+    *,
+    discrete_only: bool = False,
+) -> dict[str, Any]:
     """Map unified `colors` parameter to appropriate Plotly px_kwargs.
 
     Direct color_* kwargs take precedence and trigger a warning if
@@ -260,6 +271,10 @@ def resolve_colors(colors: Colors, px_kwargs: dict[str, Any]) -> dict[str, Any]:
     Args:
         colors: Unified color specification (str, list, dict, or None).
         px_kwargs: Existing kwargs to pass to Plotly Express.
+        discrete_only: If True, continuous colorscale strings are sampled
+            into a discrete sequence instead of setting
+            ``color_continuous_scale``.  Use for chart types that only
+            accept discrete color parameters (line, bar, area, box, pie).
 
     Returns:
         Updated px_kwargs with color parameters injected.
@@ -284,6 +299,9 @@ def resolve_colors(colors: Colors, px_kwargs: dict[str, Any]) -> dict[str, Any]:
         # Check if it's a qualitative (discrete) palette name
         if colors in _get_qualitative_scale_names():
             px_kwargs["color_discrete_sequence"] = getattr(px.colors.qualitative, colors)
+        elif discrete_only:
+            # Sample from continuous scale into a discrete sequence
+            px_kwargs["color_discrete_sequence"] = _sample_colorscale(colors)
         else:
             # Assume continuous scale
             px_kwargs["color_continuous_scale"] = colors

xarray_plotly/plotting.py

@@ -82,7 +82,7 @@ def line(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
+    px_kwargs = resolve_colors(colors, px_kwargs, discrete_only=True)
     slots = assign_slots(
         list(darray.dims),
         "line",
@@ -329,7 +329,7 @@ def fast_bar(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
+    px_kwargs = resolve_colors(colors, px_kwargs, discrete_only=True)
     slots = assign_slots(
         list(darray.dims),
         "fast_bar",
@@ -410,7 +410,7 @@ def area(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
+    px_kwargs = resolve_colors(colors, px_kwargs, discrete_only=True)
     slots = assign_slots(
         list(darray.dims),
         "area",
@@ -487,7 +487,7 @@ def box(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
+    px_kwargs = resolve_colors(colors, px_kwargs, discrete_only=True)
     slots = assign_slots(
         list(darray.dims),
         "box",
@@ -746,7 +746,7 @@ def pie(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
+    px_kwargs = resolve_colors(colors, px_kwargs, discrete_only=True)
     slots = assign_slots(
         list(darray.dims),
         "pie",