Only show a legend when the matplotlib figure has one

plotly/matplotlylib/renderer.py

@@ -78,6 +78,10 @@ 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.mpl_x_bounds, self.mpl_y_bounds = mpltools.get_axes_bounds(fig)
         margin = go.layout.Margin(
@@ -421,13 +425,14 @@ 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 isinstance(label, str) and label.startswith("_"):
+                label = None
             marked_line = go.Scatter(
                 mode=mode,
-                name=(
-                    str(props["label"])
-                    if isinstance(props["label"], str)
-                    else props["label"]
-                ),
+                name=label,
                 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

@@ -84,3 +84,12 @@ def test_multiple_traces_native_legend():
     assert plotly_fig.data[0].mode == "lines"
     assert plotly_fig.data[1].mode == "markers"
     assert plotly_fig.data[2].mode == "lines+markers"
+
+
+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)