Matplotlib 图形颜色(背景颜色)

有人能解释一下为什么下面的代码在设置图形的颜色时不起作用吗？

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)指定图形的高度和宽度时，这些都由命令 plt.savefig("trial_fig.png")提取。但是，不会选择颜色设置。为什么？

谢谢你的帮助。