Fix px opacity values misaligned when points are split into traces

CHANGELOG.md

@@ -17,6 +17,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix Plotly Express mutating lists passed to the `x` or `y` arguments in wide mode [[#5727](https://github.com/plotly/plotly.py/pull/5727)], with thanks to @cpruijsen 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 misaligning per-point `opacity` values with their markers when the data is split into several traces (e.g. by `color`, `symbol` or facets); each trace now gets the opacities of its own points, and `opacity` also accepts a column name [[#5825](https://github.com/plotly/plotly.py/pull/5825)], with thanks to @shivenaggarwal for the contribution!
 
 ## [7.1.0] - 2026-09-15
 

plotly/express/_core.py

@@ -1,6 +1,7 @@
 import plotly.graph_objs as go
 import plotly.io as pio
 from collections import namedtuple, OrderedDict
+from numbers import Number
 from ._special_inputs import IdentityMap, Constant, Range
 from .trendline_functions import ols, lowess, rolling, expanding, ewm
 
@@ -44,7 +45,11 @@
     "pattern_shape",  # renamed to marker.pattern.shape in infer_config
 ]
 all_attrables = (
-    direct_attrables + array_attrables + group_attrables + renameable_group_attrables
+    direct_attrables
+    + array_attrables
+    + group_attrables
+    + renameable_group_attrables
+    + ["opacity"]  # only refers to data when it isn't a number, see infer_config
 )
 
 cartesians = [go.Scatter, go.Scattergl, go.Bar, go.Funnel, go.Box, go.Violin]
@@ -352,6 +357,10 @@ def make_trace_kwargs(args, trace_spec, trace_data, mapping_labels, sizeref):
                 trace_patch["marker"]["sizemode"] = "area"
                 trace_patch["marker"]["sizeref"] = sizeref
                 mapping_labels[attr_label] = "%{marker.size}"
+            elif attr_name == "opacity":
+                if "marker" not in trace_patch:
+                    trace_patch["marker"] = dict()
+                trace_patch["marker"]["opacity"] = trace_data.get_column(attr_value)
             elif attr_name == "marginal_x":
                 if trace_spec.constructor == go.Histogram and args.get("z") is None:
                     mapping_labels["count"] = "%{y}"
@@ -1313,6 +1322,8 @@ def process_args_into_dataframe(
             length = len(df_output[next(iter(df_output))]) if len(df_output) else 0
             if argument is None:
                 continue
+            if field_name == "opacity" and isinstance(argument, Number):
+                continue  # a number is one opacity for all marks, not a column
             col_name = None
             # Case of multiindex
             if is_pd_like and isinstance(argument, native_namespace.MultiIndex):
@@ -2373,6 +2384,10 @@ def infer_config(args, constructor, trace_patch, layout_patch):
             go.Funnelarea,
         ]:
             trace_patch["opacity"] = args["opacity"]
+        elif isinstance(args["opacity"], str):
+            # values from a column are split along with the rest of the data,
+            # so that each trace gets the opacities of its own marks
+            attrs.append("opacity")
         else:
             trace_patch["marker"] = dict(opacity=args["opacity"])
     if (

tests/test_optional/test_px/test_px.py

@@ -382,6 +382,61 @@ def test_marginal_ranges(backend):
     assert fig.layout.yaxis3.range is None
 
 
+@pytest.mark.parametrize(
+    "fn,kwargs",
+    [
+        (px.scatter, dict(x="x", y="y")),
+        (px.bar, dict(x="x", y="y")),
+        (px.scatter_matrix, dict(dimensions=["x", "y"])),
+    ],
+)
+def test_opacity_values_split_by_color(constructor, fn, kwargs):
+    # https://github.com/plotly/plotly.py/issues/4580
+    data = dict(
+        x=[1, 2, 3, 4, 5, 6],
+        y=[2, 3, 1, 5, 4, 6],
+        color=["a", "b", "a", "b", "a", "b"],
+    )
+    opacity = [0.1, 0.2, 0.3, 0.4, 0.5, 0.6]
+    fig = fn(constructor(data), color="color", opacity=opacity, **kwargs)
+    assert len(fig.data) == 2
+    for trace in fig.data:
+        expected = [o for o, c in zip(opacity, data["color"]) if c == trace.name]
+        assert list(trace.marker.opacity) == expected
+
+
+def test_opacity_column_split_by_facets(constructor):
+    data = dict(
+        x=[1, 2, 3, 4, 5, 6],
+        y=[2, 3, 1, 5, 4, 6],
+        row=["p", "p", "q", "q", "p", "q"],
+        col=["u", "v", "v", "u", "u", "v"],
+        alpha=[0.1, 0.2, 0.3, 0.4, 0.5, 0.6],
+    )
+    fig = px.scatter(
+        constructor(data),
+        x="x",
+        y="y",
+        facet_row="row",
+        facet_col="col",
+        opacity="alpha",
+    )
+    assert len(fig.data) == 4
+    alpha_by_x = dict(zip(data["x"], data["alpha"]))
+    for trace in fig.data:
+        assert list(trace.marker.opacity) == [alpha_by_x[x] for x in trace.x]
+
+
+@pytest.mark.parametrize("opacity", [0.5, 1, np.float64(0.25)])
+def test_opacity_scalar(opacity):
+    # a number applies to every trace, even if a column has the same name
+    df = pd.DataFrame({0: [1, 2, 3], 1: [3, 1, 2], "color": ["a", "b", "a"]})
+    fig = px.scatter(df, x=0, y=1, color="color", opacity=opacity)
+    assert len(fig.data) == 2
+    for trace in fig.data:
+        assert trace.marker.opacity == opacity
+
+
 def test_render_mode(backend):
     df = nw.from_native(px.data.gapminder(return_type=backend))
     df2007 = df.filter(nw.col("year") == 2007)