Overlaying filled contour plots

[Simon de Vet <sdevet(at)is2.dal.ca>]

I have two sets of data for a series of points.  One set indicates the
values of a certain parameter, the others are the values of another
parameter.

Is there a way to overlay two filled contour plots? I'd like, for
example, to have one of the values plotted in red, the other in blue,
with purple (or another colour) where they overlap. These contour maps
have two levels each (on and off), so I don't need to worry about shades
of red or shades of blue. I just need conours for four states: both on
(purple), both off (white), one on other off and vice versa (red and
blue).

Any ideas?


-- Simon