Some refinements

 - Added tests
 - Added legend to _grouped_hist
 - DataFrame.hist now respects kwarg 'label' if specified.

Author: rhshadrach

pandas/plotting/_matplotlib/hist.py

@@ -227,6 +227,7 @@ def _grouped_hist(
     xrot=None,
     ylabelsize=None,
     yrot=None,
+    legend=False,
     **kwargs,
 ):
     """
@@ -252,8 +253,19 @@ def _grouped_hist(
     collection of Matplotlib Axes
     """
 
+    if legend and "label" not in kwargs:
+        if isinstance(data, ABCDataFrame):
+            if column is None:
+                kwargs["label"] = data.columns
+            else:
+                kwargs["label"] = column
+        else:
+            kwargs["label"] = data.name
+
     def plot_group(group, ax):
         ax.hist(group.dropna().values, bins=bins, **kwargs)
+        if legend:
+            ax.legend()
 
     if xrot is None:
         xrot = rot
@@ -340,6 +352,7 @@ def hist_series(
             xrot=xrot,
             ylabelsize=ylabelsize,
             yrot=yrot,
+            legend=legend,
             **kwds,
         )
 
@@ -383,6 +396,7 @@ def hist_frame(
             xrot=xrot,
             ylabelsize=ylabelsize,
             yrot=yrot,
+            legend=legend,
             **kwds,
         )
         return axes
@@ -408,16 +422,17 @@ def hist_frame(
     )
     _axes = _flatten(axes)
 
+    can_set_label = "label" not in kwds
+
     for i, col in enumerate(com.try_sort(data.columns)):
         ax = _axes[i]
-        if legend and "label" not in kwds:
+        if legend and can_set_label:
             kwds["label"] = col
         ax.hist(data[col].dropna().values, bins=bins, **kwds)
         ax.set_title(col)
         ax.grid(grid)
         if legend:
             ax.legend()
-            kwds.pop("label")
 
     _set_ticks_props(
         axes, xlabelsize=xlabelsize, xrot=xrot, ylabelsize=ylabelsize, yrot=yrot

pandas/tests/plotting/test_groupby.py

@@ -2,6 +2,7 @@
 
 
 import numpy as np
+import pytest
 
 import pandas.util._test_decorators as td
 
@@ -65,3 +66,17 @@ def test_plot_kwargs(self):
 
         res = df.groupby("z").plot.scatter(x="x", y="y")
         assert len(res["a"].collections) == 1
+
+
+@td.skip_if_no_mpl
+@pytest.mark.parametrize("column", [None, "B"])
+@pytest.mark.parametrize("label", [None, "D"])
+def test_hist_with_legend(column, label):
+    df = DataFrame(np.random.randn(30, 2), columns=["A", "B"])
+    df["C"] = 15 * ["a"] + 15 * ["b"]
+    g = df.groupby("C")
+    g.hist(column=column, label=label, legend=True)
+    tm.close()
+    if column != "B":
+        g["A"].hist(label=label, legend=True)
+        tm.close()

pandas/tests/plotting/test_hist_method.py

@@ -460,3 +460,14 @@ def test_axis_share_xy(self):
 
         assert ax1._shared_y_axes.joined(ax1, ax2)
         assert ax2._shared_y_axes.joined(ax1, ax2)
+
+
+@pytest.mark.slow
+@pytest.mark.parametrize("by", [None, "C"])
+@pytest.mark.parametrize("column", [None, "B"])
+@pytest.mark.parametrize("label", [None, "D"])
+def test_hist_with_legend(by, column, label):
+    df = DataFrame(np.random.randn(30, 2), columns=["A", "B"])
+    df["C"] = 15 * ["a"] + 15 * ["b"]
+    df = df.set_index("C")
+    _check_plot_works(df.hist, by=by, column=column, label=label, legend=True)