In testing some stuff for #7151, I realized that the Cairo backend inverts images plotted using imshow.

import matplotlib as mpl
mpl.use('Cairo')
import numpy as np
import matplotlib.pyplot as plt

a = np.random.randn(3,3)

im1 = plt.imshow(a, extent = [0,1,0,1])
plt.xlim(0,2.2)
plt.ylim(0,2.2)

plt.savefig('test.png')

It doesn't really matter if saving as png or pdf, the outcome is the same. With Agg, the image appears in the lower left corner:

With Cairo, the image appears in the upper left corner:

It worked correctly with mpl 1.4.3 and is broken with 1.5.0, but bisection is next to impossible because the Cairo backend was broken for a very long time.