R from cgi and Xvfb

Hi everyone.
I'm sorry for the question maybe OT.
I'm trying to use R and Python to run some scripts via web interface.
I've successfully setup mod_python for Apache and the rpy module.
R needs X11 to use png() and jpeg() devices, so I have installed Xvfb
(X virtual framebuffer). I works correctly: if I set the DISPLAY
variable to point to this X server, rpy can create png files correctly
from command-line, but it doesn't work when the python script is run
from web browser.
I restarted Apache after setting the DISPLAY variable, but the
Traceback gives me always the same error, about being not able to open
the X11 device?

Does anyone have made it work right?
How can tell Apache to run R script and forwarding X requests to my Xvfb.

Thanks,
Giovanni