Scatterplot with the 3rd dimension = color?

Duncan Murdoch <murdoch.duncan <at> gmail.com> writes:

On 11-10-02 1:11 PM, Kerry wrote:

I have 3 columns of data and want to plot each row as a point in a
scatter plot and want one column to be represented as a color gradient
(e.g. larger  values being more red). Anyone know the command or
package for this?

It's not a particularly effective display, but here's how to do it.  Use 
rainbow(101) in place of rev(heat.colors(101)) if you like.

x <- rnorm(10)
y <- rnorm(10)
z <- rnorm(10)
colors <- rev(heat.colors(101))
zcolor <- colors[(z - min(z))/diff(range(z))*100 + 1]
plot(x,y,col=zcolor)

or

d <- data.frame(x,y,z)
library(ggplot2)
qplot(x,y,colour=z,data=d)

  I agree about the "not particularly effective display"
comment, but if you have two continuous predictors and
a continuous response you've got a tough display problem --
your choices are:

  1. use color, size, or some other graphical characteristic
(pretty far down on the "Cleveland hierarchy")
  2. use a perspective plot (hard to get the right viewing
angle, often confusing)
  3. use coplots/small multiples/faceting (requires
discretizing one dimension)