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

[Jim Lemon]

Eric Harley wrote:
> 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.
> 
Hi Eric,

You can fill a rectangle with a "gradient" (slices of colors) using 
gradient.rect in the plotrix package. Filling an arbitrary polygon would 
be a lot harder to program. You may also be interested in the 
color.scale function that assigns colors to numerical values.

Jim