Simply replace plt.savefig with savefig_changing_font as follows:
$ python
>>> import matplotlib.pyplot as plt
>>> from savefig_changing_font import savefig_changing_font as savecf
>>> x = [1,2,3,4,5]
>>> y = [1,4,9,16,25]
>>> plt.plot(x,y,"ro-")
>>> plt.xlabel("$x$ for $f(x)=x^2$ as x label")
>>> plt.ylabel("$f(x)$ as y label")
>>> plt.annotate("Sample Text1",[1.5,18])
>>> plt.annotate("Sample Text2",[1.5,15],fontname="Arial")
>>> savecf(fmt="png",dpi=200) # default fmt is "pdf"
Then you can see following three files:
