Fix for Issue #12565 - font size on secondary_y

There may be other plotting methods that need testing.

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 axes 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 +2733,22 @@ 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):
+ 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