TST: Add test for plotting MultiIndex bar plot

A fix to issue pandas-dev#26186 revealed no tests existed about plotting a bar
plot for a MultiIndex, but a section of the user guide visualization
did. This section of the user guide is now in the test suite.

Author: nrebena

pandas/tests/plotting/test_frame.py

@@ -3354,6 +3354,33 @@ def test_bar_numeric(self):
         ticklocs = ax.xaxis.get_ticklocs()
         tm.assert_numpy_array_equal(ticklocs, index)
 
+    def test_bar_multiindex(self):
+        # Test from pandas/doc/source/user_guide/visualization.rst
+        # at section Plotting With Error Bars
+        # Related to issue GH: 26186
+
+        ix3 = pd.MultiIndex.from_arrays(
+            [
+                ["a", "a", "a", "a", "b", "b", "b", "b"],
+                ["foo", "foo", "bar", "bar", "foo", "foo", "bar", "bar"],
+            ],
+            names=["letter", "word"],
+        )
+
+        df3 = pd.DataFrame(
+            {"data1": [3, 2, 4, 3, 2, 4, 3, 2], "data2": [6, 5, 7, 5, 4, 5, 6, 5]},
+            index=ix3,
+        )
+
+        # Group by index labels and take the means and standard deviations
+        # for each group
+        gp3 = df3.groupby(level=("letter", "word"))
+        means = gp3.mean()
+        errors = gp3.std()
+
+        # No assertion we just ensure that we can plot a MultiIndex bar plot
+        means.plot.bar(yerr=errors, capsize=4)
+
 
 def _generate_4_axes_via_gridspec():
     import matplotlib.pyplot as plt