2d plot with modification of plotting symbol to indicate third dimension.

> I would like to plot 3-dimensional data on a two-dimensional 
scatter-plot.
 > Is there a way I can automatically modify the plot symbol (e.g. 
changing size or color) to indicate the value of a third variable? E.g. 
How can I plot weight vs. age and indicate the value of muscle mass for 
each value weight-age pair by making the plot point proportional to the 
subject's muscle mass?

Just set cex to the other variable.  For example,

plot(1:10, 1:10, cex=1:10)

Similarly, col and pch can be set to vector values, which are recycled 
through the points.

Duncan Murdoch