BUG: fontsize parameter of plot only affects x-axis.

[davidastephens]

import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.randn(10, 9), index=range(10))
df.plot()

Results in this plot (all the same font size):

df.plot(fontsize=4)

Results in this plot (only the x-axis font size is changed):

[jorisvandenbossche]

Closed by #8940