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 +251,20 @@ 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) -> list[str]:
+    """Sample *n* evenly-spaced colors from a named Plotly colorscale."""
+    scale = px.colors.get_colorscale(name)
+    samplepoints = [i / max(n - 1, 1) for i in range(n)]
+    result: list[str] = px.colors.sample_colorscale(scale, samplepoints)
+    return result
+
+
+def resolve_colors(
+    colors: Colors,
+    px_kwargs: dict[str, Any],
+    *,
+    n_colors: int | None = None,
+) -> dict[str, Any]:
     """Map unified `colors` parameter to appropriate Plotly px_kwargs.
 
     Direct color_* kwargs take precedence and trigger a warning if
@@ -260,6 +273,10 @@ 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.
+        n_colors: When set, continuous colorscale strings are sampled
+            into a discrete sequence of this length instead of setting
+            ``color_continuous_scale``.  Use for chart types that only
+            accept discrete color parameters (line, area, box, pie).
 
     Returns:
         Updated px_kwargs with color parameters injected.
@@ -284,6 +301,9 @@ 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 n_colors is not None:
+            # Sample from continuous scale into a discrete sequence
+            px_kwargs["color_discrete_sequence"] = _sample_colorscale(colors, n_colors)
         else:
             # Assume continuous scale
             px_kwargs["color_continuous_scale"] = colors

xarray_plotly/plotting.py

@@ -82,7 +82,6 @@ def line(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
     slots = assign_slots(
         list(darray.dims),
         "line",
@@ -94,6 +93,7 @@ def line(
         facet_row=facet_row,
         animation_frame=animation_frame,
     )
+    px_kwargs = resolve_colors(colors, px_kwargs, n_colors=darray.sizes.get(slots.get("color")))
 
     df = to_dataframe(darray)
     value_col = get_value_col(darray)
@@ -329,7 +329,6 @@ def fast_bar(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
     slots = assign_slots(
         list(darray.dims),
         "fast_bar",
@@ -339,6 +338,7 @@ def fast_bar(
         facet_row=facet_row,
         animation_frame=animation_frame,
     )
+    px_kwargs = resolve_colors(colors, px_kwargs, n_colors=darray.sizes.get(slots.get("color")))
 
     df = to_dataframe(darray)
     value_col = get_value_col(darray)
@@ -410,7 +410,6 @@ def area(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
     slots = assign_slots(
         list(darray.dims),
         "area",
@@ -421,6 +420,7 @@ def area(
         facet_row=facet_row,
         animation_frame=animation_frame,
     )
+    px_kwargs = resolve_colors(colors, px_kwargs, n_colors=darray.sizes.get(slots.get("color")))
 
     df = to_dataframe(darray)
     value_col = get_value_col(darray)
@@ -487,7 +487,6 @@ def box(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
     slots = assign_slots(
         list(darray.dims),
         "box",
@@ -498,6 +497,7 @@ def box(
         facet_row=facet_row,
         animation_frame=animation_frame,
     )
+    px_kwargs = resolve_colors(colors, px_kwargs, n_colors=darray.sizes.get(slots.get("color")))
 
     df = to_dataframe(darray)
     value_col = get_value_col(darray)
@@ -746,14 +746,14 @@ def pie(
     -------
     plotly.graph_objects.Figure
     """
-    px_kwargs = resolve_colors(colors, px_kwargs)
     slots = assign_slots(
         list(darray.dims),
         "pie",
         names=names,
         facet_col=facet_col,
         facet_row=facet_row,
     )
+    px_kwargs = resolve_colors(colors, px_kwargs, n_colors=darray.sizes.get(slots.get("names")))
 
     df = to_dataframe(darray)
     value_col = get_value_col(darray)