VIS: Fix DataFrame.plot() produces incorrect legend markers (#27808)

Author: charlesdong1991

doc/source/whatsnew/v1.0.0.rst

@@ -165,6 +165,7 @@ Plotting
 
 - Bug in :meth:`Series.plot` not able to plot boolean values (:issue:`23719`)
 -
+- Bug in :meth:`DataFrame.plot` producing incorrect legend markers when plotting multiple series on the same axis (:issue:`18222`)
 - Bug in :meth:`DataFrame.plot` when ``kind='box'`` and data contains datetime or timedelta data. These types are now automatically dropped (:issue:`22799`)
 
 Groupby/resample/rolling

pandas/plotting/_matplotlib/core.py

@@ -562,7 +562,7 @@ def _add_legend_handle(self, handle, label, index=None):
             self.legend_labels.append(label)
 
     def _make_legend(self):
-        ax, leg = self._get_ax_legend(self.axes[0])
+        ax, leg, handle = self._get_ax_legend_handle(self.axes[0])
 
         handles = []
         labels = []
@@ -571,7 +571,8 @@ def _make_legend(self):
         if not self.subplots:
             if leg is not None:
                 title = leg.get_title().get_text()
-                handles = leg.legendHandles
+                # Replace leg.LegendHandles because it misses marker info
+                handles.extend(handle)
                 labels = [x.get_text() for x in leg.get_texts()]
 
             if self.legend:
@@ -581,6 +582,7 @@ def _make_legend(self):
 
                 handles += self.legend_handles
                 labels += self.legend_labels
+
                 if self.legend_title is not None:
                     title = self.legend_title
 
@@ -592,16 +594,22 @@ def _make_legend(self):
                 if ax.get_visible():
                     ax.legend(loc="best")
 
-    def _get_ax_legend(self, ax):
+    def _get_ax_legend_handle(self, ax):
+        """
+        Take in axes and return ax, legend and handle under different scenarios
+        """
         leg = ax.get_legend()
+
+        # Get handle from axes
+        handle, _ = ax.get_legend_handles_labels()
         other_ax = getattr(ax, "left_ax", None) or getattr(ax, "right_ax", None)
         other_leg = None
         if other_ax is not None:
             other_leg = other_ax.get_legend()
         if leg is None and other_leg is not None:
             leg = other_leg
             ax = other_ax
-        return ax, leg
+        return ax, leg, handle
 
     @cache_readonly
     def plt(self):

pandas/tests/plotting/common.py

@@ -103,6 +103,28 @@ def _check_legend_labels(self, axes, labels=None, visible=True):
             else:
                 assert ax.get_legend() is None
 
+    def _check_legend_marker(self, ax, expected_markers=None, visible=True):
+        """
+        Check ax has expected legend markers
+
+        Parameters
+        ----------
+        ax : matplotlib Axes object
+        expected_markers : list-like
+            expected legend markers
+        visible : bool
+            expected legend visibility. labels are checked only when visible is
+            True
+        """
+        if visible and (expected_markers is None):
+            raise ValueError("Markers must be specified when visible is True")
+        if visible:
+            handles, _ = ax.get_legend_handles_labels()
+            markers = [handle.get_marker() for handle in handles]
+            assert markers == expected_markers
+        else:
+            assert ax.get_legend() is None
+
     def _check_data(self, xp, rs):
         """
         Check each axes has identical lines

pandas/tests/plotting/test_frame.py

@@ -1881,6 +1881,31 @@ def test_df_legend_labels(self):
         self._check_legend_labels(ax, labels=["LABEL_b", "LABEL_c"])
         assert df5.columns.tolist() == ["b", "c"]
 
+    def test_missing_marker_multi_plots_on_same_ax(self):
+        # GH 18222
+        df = pd.DataFrame(
+            data=[[1, 1, 1, 1], [2, 2, 4, 8]], columns=["x", "r", "g", "b"]
+        )
+        fig, ax = self.plt.subplots(nrows=1, ncols=3)
+        # Left plot
+        df.plot(x="x", y="r", linewidth=0, marker="o", color="r", ax=ax[0])
+        df.plot(x="x", y="g", linewidth=1, marker="x", color="g", ax=ax[0])
+        df.plot(x="x", y="b", linewidth=1, marker="o", color="b", ax=ax[0])
+        self._check_legend_labels(ax[0], labels=["r", "g", "b"])
+        self._check_legend_marker(ax[0], expected_markers=["o", "x", "o"])
+        # Center plot
+        df.plot(x="x", y="b", linewidth=1, marker="o", color="b", ax=ax[1])
+        df.plot(x="x", y="r", linewidth=0, marker="o", color="r", ax=ax[1])
+        df.plot(x="x", y="g", linewidth=1, marker="x", color="g", ax=ax[1])
+        self._check_legend_labels(ax[1], labels=["b", "r", "g"])
+        self._check_legend_marker(ax[1], expected_markers=["o", "o", "x"])
+        # Right plot
+        df.plot(x="x", y="g", linewidth=1, marker="x", color="g", ax=ax[2])
+        df.plot(x="x", y="b", linewidth=1, marker="o", color="b", ax=ax[2])
+        df.plot(x="x", y="r", linewidth=0, marker="o", color="r", ax=ax[2])
+        self._check_legend_labels(ax[2], labels=["g", "b", "r"])
+        self._check_legend_marker(ax[2], expected_markers=["x", "o", "o"])
+
     def test_legend_name(self):
         multi = DataFrame(
             randn(4, 4),