Fix px error bars ignoring opacity on categorical traces

CHANGELOG.md

@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## Unreleased
 
 ### Fixed
+- Fix Plotly Express error bars ignoring `opacity` on categorically colored traces. Error bars now use the marker/line color with the requested opacity baked in [[#4353](https://github.com/plotly/plotly.py/issues/4353)], with thanks to @00200200 for the contribution!
 - Fix concurrent first access to lazily initialized graph object properties, which could raise `ValueError("Invalid value")` [[#3441](https://github.com/plotly/plotly.py/issues/3441)], with thanks to @hb1915 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 [[#5765](https://github.com/plotly/plotly.py/issues/5765)], with thanks to @Irahan2 for the contribution!
 

plotly/express/_core.py

@@ -5,6 +5,7 @@
 from .trendline_functions import ols, lowess, rolling, expanding, ewm
 
 from _plotly_utils.basevalidators import ColorscaleValidator
+from _plotly_utils.colors import hex_to_rgb, unlabel_rgb
 from plotly.colors import qualitative, sequential
 import math
 
@@ -2529,6 +2530,53 @@ def get_groups_and_orders(args, grouper):
     return groups, orders
 
 
+def _color_with_opacity(color, opacity):
+    """Return an ``rgba()`` string mixing ``color`` with ``opacity``.
+
+    Plotly.js error bars have a ``color`` property but no ``opacity``, so the
+    marker/line opacity has to be baked into the error-bar color. Returns
+    ``None`` when ``color`` is missing or not a parseable hex/rgb string
+    (named CSS colors are left unchanged by the caller).
+    """
+    if not isinstance(color, str):
+        return None
+    color = color.strip()
+    try:
+        if color.startswith("#"):
+            r, g, b = hex_to_rgb(color)
+        elif "rgb" in color.lower():
+            r, g, b = unlabel_rgb(color)
+        else:
+            return None
+        return f"rgba({int(r)}, {int(g)}, {int(b)}, {opacity})"
+    except (TypeError, ValueError, IndexError):
+        return None
+
+
+def _apply_error_bar_opacity(trace, opacity):
+    """Copy marker/line color through to error bars, including ``opacity``.
+
+    Categorical Plotly Express traces already give error bars the marker
+    color via Plotly.js inheritance, but ``marker.opacity`` is ignored.
+    Continuous color (numeric ``marker.color`` arrays) cannot be mapped
+    onto ``error_y.color``, which is a single color in the schema.
+    """
+    color = getattr(trace.marker, "color", None)
+    if not isinstance(color, str):
+        color = getattr(trace.line, "color", None)
+    rgba = _color_with_opacity(color, opacity)
+    if rgba is None:
+        return
+    for name in ("error_x", "error_y", "error_z"):
+        err = getattr(trace, name, None)
+        if err is None:
+            continue
+        if err.array is None and err.arrayminus is None:
+            continue
+        if err.color is None:
+            err.color = rgba
+
+
 def make_figure(args, constructor, trace_patch=None, layout_patch=None):
     trace_patch = trace_patch or {}
     layout_patch = layout_patch or {}
@@ -2729,6 +2777,8 @@ def make_figure(args, constructor, trace_patch=None, layout_patch=None):
                 args, trace_spec, group, mapping_labels.copy(), sizeref
             )
             trace.update(patch)
+            if args.get("opacity") is not None:
+                _apply_error_bar_opacity(trace, args["opacity"])
             if fit_results is not None:
                 trendline_rows.append(mapping_labels.copy())
                 trendline_rows[-1]["px_fit_results"] = fit_results

tests/test_optional/test_px/test_px.py

@@ -4,6 +4,7 @@
 import pandas as pd
 import plotly.express as px
 import plotly.io as pio
+from plotly.colors import hex_to_rgb
 import narwhals.stable.v1 as nw
 import numpy as np
 import pytest
@@ -473,3 +474,73 @@ def test_no_warn_on_update_template():
     with warnings.catch_warnings():
         warnings.simplefilter("error")
         fig.update_layout(template="plotly_white")
+
+
+def test_error_bars_inherit_marker_opacity():
+    # https://github.com/plotly/plotly.py/issues/4353
+    df = px.data.iris()
+    df = df.assign(e_plus=df["sepal_width"] / 100, e_minus=df["sepal_width"] / 40)
+    opacity = 0.6
+    fig = px.scatter(
+        df,
+        x="sepal_width",
+        y="sepal_length",
+        color="species",
+        error_y="e_plus",
+        error_y_minus="e_minus",
+        opacity=opacity,
+    )
+    assert len(fig.data) == 3
+    for trace in fig.data:
+        assert trace.marker.opacity == opacity
+        r, g, b = hex_to_rgb(trace.marker.color)
+        assert trace.error_y.color == f"rgba({r}, {g}, {b}, {opacity})"
+
+
+def test_error_bars_keep_inherited_color_without_opacity():
+    # Without opacity, leave error_y.color unset so Plotly.js still inherits
+    # the marker color (the pre-fix behavior for categorical traces).
+    df = px.data.iris()
+    df = df.assign(e=df["sepal_width"] / 100)
+    fig = px.scatter(
+        df, x="sepal_width", y="sepal_length", color="species", error_y="e"
+    )
+    for trace in fig.data:
+        assert trace.error_y.color is None
+        assert trace.error_y.array is not None
+
+
+def test_error_x_bars_inherit_marker_opacity():
+    df = px.data.iris()
+    df = df.assign(e=df["sepal_width"] / 100)
+    opacity = 0.4
+    fig = px.scatter(
+        df,
+        x="sepal_width",
+        y="sepal_length",
+        color="species",
+        error_x="e",
+        opacity=opacity,
+    )
+    for trace in fig.data:
+        r, g, b = hex_to_rgb(trace.marker.color)
+        assert trace.error_x.color == f"rgba({r}, {g}, {b}, {opacity})"
+        assert trace.error_y.color is None
+
+
+def test_error_bars_opacity_with_continuous_color():
+    # error_y.color is a single color in the schema, so numeric color arrays
+    # cannot be mapped onto error bars. Opacity must not crash this path.
+    df = px.data.iris()
+    df = df.assign(e=df["sepal_width"] / 100)
+    fig = px.scatter(
+        df,
+        x="sepal_width",
+        y="sepal_length",
+        color="petal_width",
+        error_y="e",
+        opacity=0.6,
+    )
+    assert fig.data[0].marker.opacity == 0.6
+    assert fig.data[0].error_y.array is not None
+    assert fig.data[0].error_y.color is None