VIS: Fix DataFrame.plot() produces incorrect legend markers when plotting multiple series on the same axis

doc/source/whatsnew/v1.0.0.rst

@@ -164,7 +164,7 @@ Plotting
 ^^^^^^^^
 
 -
--
+- Bug in :meth:`DataFrame.plot` producing incorrect legend labels when plotting multiple series on the same axis (:issue:`18222`)
 
 Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/plotting/_matplotlib/core.py

@@ -166,6 +166,15 @@ def __init__(
             setattr(self, attr, value)
 
         self.ax = ax
+
+        # set subplots to True if plt.subplots() uses and assign to ax
+        from matplotlib.axes import SubplotBase
+        try:
+            ax_type = isinstance(self.ax, SubplotBase)
+        except TypeError:
+            ax_type = isinstance(self.ax[0], SubplotBase)
+        if ax_type:
+            self.subplots = True
         self.fig = fig
         self.axes = None