Fix px marginal color with named continuous color scales

When color is continuous, Plotly Express colors marginal traces with the
first entry of color_continuous_scale. For a named scale such as
"Viridis" that entry was the first character of the string ("V"), and
for a list of [position, color] pairs it was the pair itself, both of
which are invalid marker colors. Normalize the scale with
ColorscaleValidator before taking its first color.

Closes #2059

Author: anyingiit

CHANGELOG.md

@@ -21,6 +21,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix `convert_dash` mapping dotted lines (`linestyle=':'`) to invalid `'circle'` in Plotly, correctly converting them to `'dot'` and preserving custom dash patterns as pixel lists, [[#5719](https://github.com/plotly/plotly.py/pull/5719)] with thanks to @robertoffmoura for the contribution!
 - Fix `px.sunburst`, `px.treemap` and `px.icicle` listing sectors in a different order on every run when `path` is used with a Polars DataFrame; sectors now follow their order of first appearance for all dataframe backends [[#5766](https://github.com/plotly/plotly.py/pull/5766)], with thanks to @Irahan2 for the contribution!
 - Fix `mpl_to_plotly` conversion of matplotlib contour lines and line collections: close contour rings ending with `Z` codes, support dash styles, convert date x-axes, separate disjoint subpaths with `None` separators, hide line collection traces from the legend by default, and group consecutive same-style lines into single traces [[#5770](https://github.com/plotly/plotly.py/pull/5770)], with thanks to @robertoffmoura for the contribution!
+- Fix Plotly Express marginal plots (`marginal_x`/`marginal_y`) failing with an invalid `marker.color` (such as `'V'` for `"Viridis"`) when `color` is continuous and `color_continuous_scale` is given as a named color scale or as a list of `[position, color]` pairs; the marginal traces now use the first color of the normalized scale [[#5839](https://github.com/plotly/plotly.py/pull/5839)]
 
 ## [7.1.0] - 2026-09-15
 

plotly/express/_core.py

@@ -995,7 +995,13 @@ def make_trace_spec(args, constructor, attrs, trace_patch):
             ):
                 if "marker" not in trace_spec.trace_patch:
                     trace_spec.trace_patch["marker"] = dict()
-                first_default_color = args["color_continuous_scale"][0]
+                # color_continuous_scale may be a named scale (e.g. "Viridis") or a
+                # list of [position, color] pairs, so normalize it before picking
+                # the first color
+                colorscale_validator = ColorscaleValidator("colorscale", "make_figure")
+                first_default_color = colorscale_validator.validate_coerce(
+                    args["color_continuous_scale"]
+                )[0][1]
                 trace_spec.trace_patch["marker"]["color"] = first_default_color
             result.append(trace_spec)
 

tests/test_optional/test_px/test_marginals.py

@@ -204,3 +204,29 @@ def test_unsupported_marginal_raises_clear_error():  # issue 4654
         px.scatter(x=[1, 2, 3], y=[2, 3, 4], marginal_y="density")
     with pytest.raises(ValueError, match="Supported marginal plot types"):
         px.histogram(x=[1, 2, 3], marginal="density")
+
+
+@pytest.mark.parametrize(
+    "color_continuous_scale, expected_color",
+    [
+        ("Viridis", px.colors.sequential.Viridis[0]),
+        ("magma", px.colors.sequential.Magma[0]),
+        ("Viridis_r", px.colors.sequential.Viridis[-1]),
+        (["red", "blue"], "red"),
+        ([[0, "red"], [1, "blue"]], "red"),
+    ],
+)
+@pytest.mark.parametrize("marginal", ["histogram", "box", "violin", "rug"])
+def test_marginal_color_with_continuous_color_scale(
+    color_continuous_scale, expected_color, marginal
+):  # issue 2059
+    fig = px.scatter(
+        x=[1, 2, 3],
+        y=[2, 3, 4],
+        color=[1, 2, 3],
+        marginal_x=marginal,
+        marginal_y=marginal,
+        color_continuous_scale=color_continuous_scale,
+    )
+    assert fig.data[1].marker.color == expected_color
+    assert fig.data[2].marker.color == expected_color