X11 is not availble

Hi there,

after several problems with installing R on a Debian server It now  
finally works at least roughly. With roughly I mean that the 'core'  
function do work properly, but I'm still having problems with the  
image export. When I try to call

png(filename="my_file.png", width=800, height=300, pointsize=12, bg =  
"white")

I just get the error message "X11 is not available". But I don't know  
what packages (or symlinks???) could still be missing.
I've installed R using the latest 2.8.0 version from the server which  
I could compile (after solving quite some different problems) normally.

I hope anyone can help me out, best wishes,

Dennis Schmidt