[mod_python] Matplotlib error from a mod_python script "root is not a writable dir... "

I have successfully configured mod_python and using some python scripts
am accessing a database for the purpose of generating plots using
matplotlib. 

The problem is that when I try to save a plot to a subdir of the Apache
document root I get the following error. 

'/root' is not a writable dir; you must set the environment variable
HOME to be a writable dir 

echo $HOME  returns '/root' but setting it to the correct path and even
changing the dir permissions does no good. 

Tried useing the MPLCONFIGDIR to point to a MATPLOTLIBRC file but no
luck there either. 

Does anyone know What HOME variable is this error message referring to?

BTW Graham, I came across your article on "modwsgi" though it may not be
directly related I will read it in hopes it gives me some insight.  

All help appreciated. 

Thanks 

Ron