ENH: Add UserWarning when plotting bar plot with MultiIndex

Author: nrebena

pandas/plotting/_matplotlib/core.py

@@ -1319,6 +1319,9 @@ def __init__(self, data, **kwargs):
 
         if isinstance(self.data.index, ABCMultiIndex):
             # No real handling for MultiIndex yet
+            warnings.warn(
+                "Bar plot with a MultiIndex is not supported.", UserWarning,
+            )
             self.ax_index = np.arange(len(data))
         else:
             self.ax_index = self.data.index