有人可以解释一下设置图形的面色时为什么下面的代码不起作用吗？

import matplotlib.pyplot as plt

# create figure instance
fig1 = plt.figure(1)
fig1.set_figheight(11)
fig1.set_figwidth(8.5)

rect = fig1.patch
rect.set_facecolor('red') # works with plt.show().  
                          # Does not work with plt.savefig("trial_fig.png")

ax = fig1.add_subplot(1,1,1)

x = 1, 2, 3
y = 1, 4, 9
ax.plot(x, y)

# plt.show()  # Will show red face color set above using rect.set_facecolor('red')

plt.savefig("trial_fig.png") # The saved trial_fig.png DOES NOT have the red facecolor.

# plt.savefig("trial_fig.png", facecolor='red') # Here the facecolor is red.

当我使用fig1.set_figheight(11)
fig1.set_figwidth(8.5)这些命令指定图形的高度和宽度时，这些命令将被拾取plt.savefig("trial_fig.png")。但是，没有选择Facecolor设置。为什么？

谢谢你的帮助。