figure: facecolor not respected (Agg backend) ?

Hi,

i am using the simple_plot.py example with the Agg or WXAgg backends.
If i insert the following line:
rcParams["figure.facecolor"] = "r"
the background is red when using the WXAgg driver but with the Agg backend the background of the png image is still white.

Any hints ?
(matplotlib 0.64, linux)

greetings
  Uwe