Using python 3.5.1, matplotlib 1.5.1, OpenSuse 13.2

I am getting odd behaviour using imshow alpha with the pgf backend. The following code produces an image and a colorbar, the alpha matches with the agg backend.

import matplotlib
matplotlib.use('agg')
import matplotlib.pyplot as plt
fg, ax = plt.subplots()
cb = ax.imshow([[0, 1], [0.5, 0.5]], alpha=0.5)
fg.colorbar(cb)
plt.savefig('pic.png')

Using the pgf backend, I lose the alpha on the image but not the colorbar.

I want to write my plot as a .pgf file but that requires pngs for the colour bar and the image, so I lose alpha for my nice latex pgf files.

Has anybody else had this problem?