DOC: Fix validation error RT01 in pandas/plotting (pandas-dev#25356)

Author: ihsansecer

pandas/plotting/_core.py

@@ -2477,6 +2477,11 @@ def hist_series(self, by=None, ax=None, grid=True, xlabelsize=None,
     `**kwds` : keywords
         To be passed to the actual plotting function
 
+    Returns
+    -------
+    axes: matplotlib.AxesSubplot
+        A histogram plot.
+
     See Also
     --------
     matplotlib.axes.Axes.hist : Plot a histogram using matplotlib.

pandas/plotting/_misc.py

@@ -44,6 +44,11 @@ def scatter_matrix(frame, alpha=0.5, figsize=None, ax=None, grid=False,
     kwds : other plotting keyword arguments
         To be passed to scatter function
 
+    Returns
+    -------
+    axes: numpy.ndarray
+        A matrix of scatter plots.
+
     Examples
     --------
     >>> df = pd.DataFrame(np.random.randn(1000, 4), columns=['A','B','C','D'])