[Gnuplot-info] an easy way to paint something on the base of a 3d plot?

[<sa...@cr...>]

i'd like to depict the domain of a 2 parameters function by drawing it
in a coloured shape on the basis
(strictly speaking it is not a function but let me simplify)...

i was thinking of having some flat horizontal 3d region defined only out
of the domain, with surface unset and pm2d on the basis, so that i would
get it's horizontal projection drawn in some color on the base...

is there anything less tricky and more efficient?

thank you
alessandro