[R] is it possible to fill with a color or transparency gradient?

[Eric Harley]

Hi all,

Is there a way to fill a rectangle or polygon with a color and/or
transparency gradient?  This would be extremely useful for me in terms
of adding some additional information to some plots I'm making,
especially if I could define the gradient on my own by putting
functions into rgb something like rgb( r=f(x,y), g=f(x,y), b=f(x,y),
alpha=f(x,y) ).  Not so important whether the coordinates are in terms
of the plot axes or normalized to the polygon itself somehow.  Ideally
it would work not only for a fill color but also for shading lines.

I haven't been using R very long, so it's possible that I'm just
missing something, but I haven't found anything like this in the help
files.  I've tried to poke around in graphics, grid, and ggplot,
without any luck so far.  I really like some of the functionality in
ggplot, and it does some nice things with continuous gradients for the
color of scatter plot points, for example, but it each individual
point (or grob) is always one solid color as far as I can tell.

Thanks,
Eric