irregual space plot

Hello everyone...

I was hoping an R wizard could help me out with this one.  My current R
programming endeavors require that I generate plots of groundwater state
variables (e.g. pressure fields, contaminate plumes, etc.) in a 2-D slice of
a 3-D system.  While the functions contour() and filled.contour() are close
to what I?m looking for, they unfortunately lack in their inability to deal
with irregular boundaries (non-rectangular systems).  The irregular systems
I?m working with are made up of finite elements (each 2-D element
constructed from 3 or 4 nodes) where linear basis functions describe every
point (x,y) in the element.  I believe I could deal with my plotting dilemma
if I had a function very similar to ?.Internal(filledcontour())?.  A
function, which would allow me to specify contours or colors between
contours within an element.  If someone could give me a hand on this project
or point me towards helpful resources, it would be greatly appreciated.

Regards,

Jason

PS:  If any additional information is needed, please ask.