Keep overall trendline off marginal subplots

CHANGELOG.md

@@ -7,6 +7,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 - 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!
+- Fix `trendline_scope="overall"` drawing the overall trendline on the `marginal_x`/`marginal_y` subplots as well as the main plot; it is now drawn only on the main subplots [[#4669](https://github.com/plotly/plotly.py/issues/4669)]
 
 
 ## [7.1.0] - 2026-09-15

plotly/express/_core.py

@@ -2833,9 +2833,13 @@ def make_figure(args, constructor, trace_patch=None, layout_patch=None):
             args, trendline_spec, args["data_frame"], {}, sizeref
         )
         trendline_trace.update(patch)
-        fig.add_trace(
-            trendline_trace, row="all", col="all", exclude_empty_subplots=True
-        )
+        main_rows = nrows - 1 if args.get("marginal_x") is not None else nrows
+        main_cols = ncols - 1 if args.get("marginal_y") is not None else ncols
+        for row in range(1, main_rows + 1):
+            for col in range(1, main_cols + 1):
+                fig.add_trace(
+                    trendline_trace, row=row, col=col, exclude_empty_subplots=True
+                )
         fig.update_traces(selector=-1, showlegend=True)
         if fit_results is not None:
             trendline_rows.append(dict(px_fit_results=fit_results))

tests/test_optional/test_px/test_trendline.py

@@ -264,3 +264,34 @@ def test_overall_trendline(backend):
     params3 = results3["px_fit_results"].iloc[0].params
 
     assert np.all(np.array_equal(params1, params3))
+
+
+@pytest.mark.parametrize(
+    "kwargs",
+    [
+        dict(marginal_x="box", marginal_y="box"),
+        dict(marginal_x="histogram", facet_col="smoker"),
+        dict(marginal_y="violin", facet_row="smoker"),
+    ],
+)
+def test_overall_trendline_skips_marginals(backend, kwargs):
+    df = px.data.tips(return_type=backend)
+    fig = px.scatter(
+        df,
+        x="total_bill",
+        y="tip",
+        color="sex",
+        trendline="ols",
+        trendline_scope="overall",
+        **kwargs,
+    )
+    trendline_axes = [
+        (t.xaxis, t.yaxis) for t in fig.data if t.name == "Overall Trendline"
+    ]
+    scatter_axes = {
+        (t.xaxis, t.yaxis)
+        for t in fig.data
+        if t.type == "scatter" and t.name != "Overall Trendline"
+    }
+    assert len(trendline_axes) == len(scatter_axes)
+    assert set(trendline_axes) == scatter_axes