Matplotlib figure facecolor (background color)

Can someone please explain why the code below does not work when setting the facecolor of the figure?

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.

When I specify the height and width of the figure using fig1.set_figheight(11) fig1.set_figwidth(8.5) these are picked up by the command plt.savefig("trial_fig.png"). However, the facecolor setting is not picked up. Why?

Thanks for your help.