plotting data in non-orthogonal coords.

Hi !
I am wondering how to plot data (e.g.  f(x,y) ) in a coordinate system spanned 
by two non-orthogonal basis vectors (e.g. hexagonal symmetry). The data is 
given on an equally spaced grid in theses coords and i would like to do a 
contour plot (e.g. with filled.contour).

Thanks for your help. Andreas