python之如何设置 ax.legend 字体大小

这是我的代码

import os,sys 
import Image 
import matplotlib.pyplot as plt 
from matplotlib.pyplot import * 
from matplotlib.font_manager import FontProperties 
 
jpgfile = Image.open("t002.jpg") 
 
# Set up the figure and axes. 
fig = plt.figure(figsize=(18,10))  # ...or whatever size you want. 
ax = fig.add_subplot(111) 
ax.legend(fontsize=18) 
# Draw things. 
plt.imshow(jpgfile)  # Unlike plot and scatter, not a method on ax. 
ax.set_xlabel('normalized resistivities') 
ax.set_ylabel('normalized velocities') 
ax.set_xticks([]); ax.set_yticks([]) 
 
# Save and show. 
plt.savefig("fig.jpg") 
plt.show() 

但是

/usr/local/lib/python2.7/dist-packages/matplotlib/axes/_axes.py:519: UserWarning: No labelled objects found. Use label='...' kwarg on individual plots 

如何设置标签？