plotly · lucasjamar · Aug 16, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Added
+- Support `marginal_x`/`marginal_y="heatmap"` in `density_heatmap`/`density_contour`, drawing a single-row/column heatmap strip in the margin colored by the same `z`/`histfunc` aggregate as the main plot [[#5706](https://github.com/plotly/plotly.py/issues/5706)]
+
 ### Fixed
 - Fix `hex_to_rgb` parsing of 3-digit shorthand hexadecimal colors such as `#FFF` [[#5662](https://github.com/plotly/plotly.py/pull/5662)], with thanks to @genrichez for the contribution!
 - Add `<!doctype html>` to the `to_html()` template to comply with modern web standards [[#5693](https://github.com/plotly/plotly.py/pull/5693)], with thanks to @mishrakushal for the contribution!

diff --git a/plotly/express/_chart_types.py b/plotly/express/_chart_types.py
@@ -142,6 +142,14 @@ def density_contour(
             "For `density_heatmap` and `density_contour` these values are used as the inputs to `histfunc`.",
         ],
         histfunc=["The arguments to this function are the values of `z`."],
+        marginal_x=[
+            "Also supports `'heatmap'`, showing a single-row heatmap colored by the aggregate value. "
+            "Uses a default colorscale since `density_contour` has no `color_continuous_scale` argument.",
+        ],
+        marginal_y=[
+            "Also supports `'heatmap'`, showing a single-column heatmap colored by the aggregate value. "
+            "Uses a default colorscale since `density_contour` has no `color_continuous_scale` argument.",
+        ],
     ),
 )
 
@@ -214,6 +222,12 @@ def density_heatmap(
         histfunc=[
             "The arguments to this function are the values of `z`.",
         ],
+        marginal_x=[
+            "Also supports `'heatmap'`, showing a single-row heatmap colored by the aggregate value.",
+        ],
+        marginal_y=[
+            "Also supports `'heatmap'`, showing a single-column heatmap colored by the aggregate value.",
+        ],
     ),
 )
 

diff --git a/plotly/express/_core.py b/plotly/express/_core.py
@@ -971,12 +971,48 @@ def make_trace_spec(args, constructor, attrs, trace_patch):
                     ),
                     marginal=letter,
                 )
+            elif args["marginal_" + letter] == "heatmap":
+                if constructor not in [go.Histogram2d, go.Histogram2dContour]:
+                    raise ValueError(
+                        "`marginal_x`/`marginal_y` value `'heatmap'` is only supported "
+                        "for `density_heatmap` and `density_contour`."
+                    )
+                other_letter = "y" if letter == "x" else "x"
+                heatmap_trace_patch = dict(
+                    coloraxis="coloraxis1", histfunc=args.get("histfunc"), **axis_map
+                )
+                # `nbinsx`/`nbinsy` are only a target bin count -- plotly.js's "nice
+                # number" bin-sizing can still round to more than one bin. Force
+                # exactly one bin by setting explicit bin edges covering the data.
+                other_col = args["data_frame"].get_column(args[other_letter])
+                other_min = nw.to_py_scalar(other_col.min())
+                other_max = nw.to_py_scalar(other_col.max())
+                span = (other_max - other_min) or 1
+                pad = span * 0.001
+                other_bins = dict(
+                    start=other_min - pad, end=other_max + pad, size=span + 2 * pad
+                )
+                if letter == "x":
+                    heatmap_trace_patch["xbingroup"] = "x"
+                    heatmap_trace_patch["ybins"] = other_bins
+                else:
+                    heatmap_trace_patch["ybingroup"] = "y"
+                    heatmap_trace_patch["xbins"] = other_bins
+                trace_spec = TraceSpec(
+                    constructor=go.Histogram2d,
+                    attrs=[letter, other_letter, "z"],
+                    trace_patch=heatmap_trace_patch,
+                    marginal=letter,
+                )
             else:
                 raise ValueError(
                     f"Invalid value '{args['marginal_' + letter]}' for `marginal_{letter}`. "
-                    "Supported marginal plot types are: 'rug', 'box', 'violin', 'histogram'."
+                    "Supported marginal plot types are: "
+                    "'rug', 'box', 'violin', 'histogram', 'heatmap'."
                 )
-            if "color" in attrs or "color" not in args:
+            if trace_spec.constructor != go.Histogram2d and (
+                "color" in attrs or "color" not in args
+            ):
                 if "marker" not in trace_spec.trace_patch:
                     trace_spec.trace_patch["marker"] = dict()
                 first_default_color = args["color_continuous_scale"][0]
@@ -2337,6 +2373,10 @@ def infer_config(args, constructor, trace_patch, layout_patch):
     if constructor in [go.Histogram2d, go.Densitymap, go.Densitymapbox]:
         show_colorbar = True
         trace_patch["coloraxis"] = "coloraxis1"
+    elif constructor == go.Histogram2dContour and (
+        args.get("marginal_x") == "heatmap" or args.get("marginal_y") == "heatmap"
+    ):
+        show_colorbar = True
 
     if "opacity" in args:
         if args["opacity"] is None:
@@ -2630,6 +2670,10 @@ def make_figure(args, constructor, trace_patch=None, layout_patch=None):
                             trace_spec.constructor in [go.Histogram]
                             and m.variable in ["symbol", "dash"]
                         )
+                        or (
+                            trace_spec.constructor == go.Histogram2d
+                            and m.variable in ["symbol", "pattern", "dash", "color"]
+                        )
                     ):
                         pass
                     elif (
@@ -2727,17 +2771,18 @@ def make_figure(args, constructor, trace_patch=None, layout_patch=None):
     if show_colorbar:
         colorvar = (
             "z"
-            if constructor in [go.Histogram2d, go.Densitymap, go.Densitymapbox]
+            if constructor
+            in [go.Histogram2d, go.Histogram2dContour, go.Densitymap, go.Densitymapbox]
             else "color"
         )
-        range_color = args["range_color"] or [None, None]
+        range_color = args.get("range_color") or [None, None]
 
         colorscale_validator = ColorscaleValidator("colorscale", "make_figure")
         coloraxis_dict = dict(
             colorscale=colorscale_validator.validate_coerce(
-                args["color_continuous_scale"]
+                args.get("color_continuous_scale")
             ),
-            cmid=args["color_continuous_midpoint"],
+            cmid=args.get("color_continuous_midpoint"),
             cmin=range_color[0],
             cmax=range_color[1],
             colorbar=dict(

diff --git a/tests/test_optional/test_px/test_marginals.py b/tests/test_optional/test_px/test_marginals.py
@@ -26,6 +26,87 @@ def test_single_marginals(backend, px_fn, marginal, orientation):
     assert len(fig.data) == 1 + (marginal is not None)
 
 
+@pytest.mark.parametrize("px_fn", [px.density_heatmap, px.density_contour])
+def test_marginal_heatmap_uses_z_and_histfunc(backend, px_fn):
+    df = px.data.tips(return_type=backend)
+    # backend-independent reference for min/max, since e.g. pyarrow columns don't
+    # support .min()/.max() directly
+    pdf = px.data.tips()
+
+    fig = px_fn(
+        df,
+        x="total_bill",
+        y="tip",
+        z="size",
+        histfunc="sum",
+        marginal_x="heatmap",
+        marginal_y="heatmap",
+    )
+    assert len(fig.data) == 3
+    marginal_x_trace, marginal_y_trace = fig.data[1], fig.data[2]
+
+    assert marginal_x_trace.type == "histogram2d"
+    assert marginal_x_trace.coloraxis == "coloraxis"
+    assert marginal_x_trace.histfunc == "sum"
+    # a single bin covering the full y range, so the strip is exactly one row
+    assert marginal_x_trace.ybins.start <= pdf["tip"].min()
+    assert marginal_x_trace.ybins.end >= pdf["tip"].max()
+    assert marginal_x_trace.ybins.size >= pdf["tip"].max() - pdf["tip"].min()
+
+    assert marginal_y_trace.type == "histogram2d"
+    assert marginal_y_trace.coloraxis == "coloraxis"
+    assert marginal_y_trace.histfunc == "sum"
+    # a single bin covering the full x range, so the strip is exactly one column
+    assert marginal_y_trace.xbins.start <= pdf["total_bill"].min()
+    assert marginal_y_trace.xbins.end >= pdf["total_bill"].max()
+    assert marginal_y_trace.xbins.size >= pdf["total_bill"].max() - pdf["total_bill"].min()
+
+    assert fig.layout.coloraxis.colorbar.title.text == "sum of size"
+
+
+@pytest.mark.parametrize("px_fn", [px.density_heatmap, px.density_contour])
+def test_marginal_heatmap_without_z(backend, px_fn):
+    df = px.data.tips(return_type=backend)
+
+    fig = px_fn(
+        df, x="total_bill", y="tip", marginal_x="heatmap", marginal_y="heatmap"
+    )
+    marginal_x_trace, marginal_y_trace = fig.data[1], fig.data[2]
+
+    assert marginal_x_trace.type == "histogram2d"
+    assert marginal_x_trace.coloraxis == "coloraxis"
+    assert marginal_x_trace.histfunc is None
+
+    assert marginal_y_trace.type == "histogram2d"
+    assert marginal_y_trace.coloraxis == "coloraxis"
+    assert marginal_y_trace.histfunc is None
+
+    assert fig.layout.coloraxis.colorbar.title.text == "count"
+
+
+def test_marginal_heatmap_unsupported_chart_type_raises():
+    with pytest.raises(ValueError, match="only supported for `density_heatmap`"):
+        px.scatter(x=[1, 2, 3], y=[2, 3, 4], marginal_x="heatmap")
+    with pytest.raises(ValueError, match="only supported for `density_heatmap`"):
+        px.scatter(x=[1, 2, 3], y=[2, 3, 4], marginal_y="heatmap")
+    with pytest.raises(ValueError, match="only supported for `density_heatmap`"):
+        px.histogram(x=[1, 2, 3], marginal="heatmap")
+
+
+def test_marginal_heatmap_with_discrete_color(backend):  # density_contour + line.color
+    df = px.data.tips(return_type=backend)
+    fig = px.density_contour(
+        df, x="total_bill", y="tip", color="sex", marginal_x="heatmap"
+    )
+    assert len(fig.data) == 4
+    assert [t.type for t in fig.data] == [
+        "histogram2dcontour",
+        "histogram2d",
+        "histogram2dcontour",
+        "histogram2d",
+    ]
+
+
 def test_unsupported_marginal_raises_clear_error():  # issue 4654
     # An unsupported marginal type used to fail deep inside make_figure with a
     # cryptic "'NoneType' object has no attribute 'constructor'". It should