Only show a legend when the matplotlib figure has one

CHANGELOG.md

@@ -9,6 +9,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix concurrent first access to lazily initialized graph object properties, which could raise `ValueError("Invalid value")` [[#5691](https://github.com/plotly/plotly.py/pull/5691)], with thanks to @hb1915 for the contribution!
 - Fix `mpl_to_plotly` crashing on touching bars (such as `plt.hist`) due to floating-point noise producing negative `bargap` values by clamping `bargap` to `[0, 1]` [[#5696](https://github.com/plotly/plotly.py/pull/5696)], with thanks to @robertoffmoura for the contribution!
 - Fix `mpl_to_plotly` drawing step plots with diagonal lines instead of vertical/horizontal step segments by mapping matplotlib step drawstyles (`steps-pre`, `steps-post`, `steps-mid`) to plotly line shapes [[#5697](https://github.com/plotly/plotly.py/pull/5697)], with thanks to @robertoffmoura for the contribution!
+- Fix `mpl_to_plotly` showing unwanted legends with placeholder or internal label entries (`_nolegend_`, `_childN`) by defaulting `layout.showlegend=False` unless the matplotlib figure explicitly defines a legend [[#5699](https://github.com/plotly/plotly.py/pull/5699)], with thanks to @robertoffmoura for the contribution!
 - Export tick label colors to the plotly tickfont in `mpl_to_plotly` [[#5716](https://github.com/plotly/plotly.py/pull/5716)], 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!
 

plotly/matplotlylib/renderer.py

@@ -91,6 +91,10 @@ def open_figure(self, fig, props):
             height=int(props["figheight"] * props["dpi"]),
             autosize=False,
             hovermode="closest",
+            # plotly.js auto-names unnamed traces "trace N" and shows them
+            # in the legend; the legend is only enabled when the mpl figure
+            # actually has one (see open_legend)
+            showlegend=False,
         )
         self.plotly_fig["layout"].paper_bgcolor = _export_color(props["figbg"])
         self.mpl_x_bounds, self.mpl_y_bounds = mpltools.get_axes_bounds(fig)
@@ -485,13 +489,18 @@ def draw_marked_line(self, **props):
                     ),
                 )
         if props["coordinates"] == "data":
+            label = props["label"]
+            # matplotlib uses "_nolegend_" and auto-generated "_childN"
+            # labels for artists that must not appear in a legend
+            if not label or (isinstance(label, str) and label.startswith("_")):
+                label = None
+                showlegend = False
+            else:
+                showlegend = None
             marked_line = go.Scatter(
                 mode=mode,
-                name=(
-                    str(props["label"])
-                    if isinstance(props["label"], str)
-                    else props["label"]
-                ),
+                name=label,
+                showlegend=showlegend,
                 x=[xy_pair[0] for xy_pair in props["data"]],
                 y=[xy_pair[1] for xy_pair in props["data"]],
                 xaxis="x{0}".format(self.axis_ct),

plotly/matplotlylib/tests/test_renderer.py

@@ -527,6 +527,34 @@ def test_fixed_formatter_ticktext():
     assert plotly_fig.layout.xaxis.ticktext == ("Baseline", "param = 1", "param = 2")
 
 
+def test_no_legend_entries_for_internal_mpl_labels():
+    """mpl internal labels (_nolegend_, _childN) must not become legend entries."""
+    fig, ax = plt.subplots()
+    ax.plot([0, 1, 2, 3], [0, 1, 0, 1], "b", [0, 1, 2, 3], [1, 0, 1, 0], "r--")
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.showlegend == False
+    assert all(t.name is None for t in plotly_fig.data)
+
+
+def test_unlabeled_traces_hidden_from_legend_when_figure_has_legend():
+    """Traces without labels must have showlegend=False when a figure has a legend."""
+    fig, ax = plt.subplots()
+    ax.plot([0, 1], [0, 1], label="Labeled line")
+    ax.plot([0, 1], [1, 0])  # Unlabeled line
+    ax.legend()
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    assert plotly_fig.layout.showlegend == True
+    assert len(plotly_fig.data) == 2
+    assert plotly_fig.data[0].name == "Labeled line"
+    assert plotly_fig.data[0].showlegend is not False
+    assert plotly_fig.data[1].name is None
+    assert plotly_fig.data[1].showlegend is False
+
+
 def test_custom_date_xtickvals_are_converted():
     """Custom tick values on a date axis must be converted to date strings,
     not left as raw matplotlib date numbers or datetime objects."""