You have the sources (for each), so why not read them?
The Unix X11 device has this helvetica hardcoded: what has arial to do 
with it?   If you would like more flexibility, please supply a patch 
against the current R-devel sources.
On Fri, 29 Aug 2003, M.Kondrin wrote:
> Can I somehow set x11 device to use font different from default 
> helvetica-arial (times for example?)? Is this font hardcoded into R or 
> can I substitute some other system font? Does x11 device use 
> standard/user-supplied .Xresources file (under Linux)?
> The questions are the same for gtk() device (with .gtkrc substituted for 
> .Xresources).
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595