BUG: Set secondary axis font size for secondary_y during plotting

The parameter was not being respected for `secondary_y`.

Closes gh-12565

Author: Morgan243

doc/source/whatsnew/v0.21.0.txt

@@ -166,7 +166,7 @@ I/O
 
 Plotting
 ^^^^^^^^
-
+- Bug in plotting methods using ``secondary_y`` and ``fontsize`` not setting secondary axis font size (:issue:`12565`)
 
 
 Groupby/Resample/Rolling

pandas/plotting/_core.py

@@ -379,13 +379,22 @@ def _post_plot_logic_common(self, ax, data):
             self._apply_axis_properties(ax.xaxis, rot=self.rot,
                                         fontsize=self.fontsize)
             self._apply_axis_properties(ax.yaxis, fontsize=self.fontsize)
+
+            if hasattr(ax, 'right_ax'):
+                self._apply_axis_properties(ax.right_ax.yaxis,
+                                            fontsize=self.fontsize)
+
         elif self.orientation == 'horizontal':
             if self._need_to_set_index:
                 yticklabels = [labels.get(y, '') for y in ax.get_yticks()]
                 ax.set_yticklabels(yticklabels)
             self._apply_axis_properties(ax.yaxis, rot=self.rot,
                                         fontsize=self.fontsize)
             self._apply_axis_properties(ax.xaxis, fontsize=self.fontsize)
+
+            if hasattr(ax, 'right_ax'):
+                self._apply_axis_properties(ax.right_ax.yaxis,
+                                            fontsize=self.fontsize)
         else:  # pragma no cover
             raise ValueError
 

pandas/tests/plotting/test_frame.py

@@ -2733,6 +2733,23 @@ def test_rcParams_bar_colors(self):
                 barplot = pd.DataFrame([[1, 2, 3]]).plot(kind="bar")
         assert color_tuples == [c.get_facecolor() for c in barplot.patches]
 
+    @pytest.mark.parametrize('method', ['line', 'barh', 'bar'])
+    def test_secondary_axis_font_size(self, method):
+        # GH: 12565
+        df = (pd.DataFrame(np.random.randn(15, 2),
+                           columns=list('AB'))
+              .assign(C=lambda df: df.B.cumsum())
+              .assign(D=lambda df: df.C * 1.1))
+
+        fontsize = 20
+        sy = ['C', 'D']
+
+        kwargs = dict(secondary_y=sy, fontsize=fontsize,
+                      mark_right=True)
+        ax = getattr(df.plot, method)(**kwargs)
+        self._check_ticks_props(axes=ax.right_ax,
+                                ylabelsize=fontsize)
+
 
 def _generate_4_axes_via_gridspec():
     import matplotlib.pyplot as plt