Mysterious R script behavior when called from webserver

Hi,

I am trying to transition a system based on dynamic image generation (via R) 
from our development system to a production environment. Our R script 
functions as expected when run by a regular user. However the script dies 
when calling the png() function, when started by the webserver user.

Here are some details

sessionInfo()

R version 2.9.2 (2009-08-24)
i686-pc-linux-gnu
locale:
C 
attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

The script gets to this line:

png(file=filename, width=600, height=400)

and then dies. It leaves an empty PNG file where it should be, however it 
never finishes the file. If 

I replace png() with pdf() an output file is generated and closed by dev.off() 
as expected.

It seems like the environment is setup just as when started by a regular user, 
specifically the LD_LIBRARY_PATH variable.

This behavior suggests that R is encountering an error, and stopping. However 
there is no reporting of the error. Is there any way to get more verbose 
error reporting?

Cheers,
Dylan

Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341