R help - Oct 2006 - is it possible to fill with a color or transparency gradient?

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

On Fri, 2006-10-06 at 16:15 -0400, 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.
> 
> Thanks,
> Eric
Take a look at the gradient.rect() function in Jim Lemon's 'plotrix'
CRAN package.

HTH,

Marc Schwartz

> 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.
ggplot supports alpha blending provided your graphics device does too
(Quartz on the mac, or pdf version 1.3 or later).  Have a look at the
alpha function for easily making transparent colours, as well as
scgradient for specifying the colour scale.

Oh, I see you want a gradient fill on a polygon - that's a lot harder,
and I can only suggest dividing your polygon up into smaller
rectangles and filling those individually.  That appears to be what
gradient.rect does.

Hadley

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