[R] Lattice: Customizing point-sizes with groups

[Paul Boutros]

Hello,

I am creating a scatter-plot in lattice, and I would like to customize  
the size of each point so that some points are larger and others  
smaller.  Here's a toy example:

library(lattice);

temp <- data.frame(
         x = 1:10,
         y = 1:10,
         cex = rep( c(1,3), 5),
         groups = c( rep("A", 5), rep("B", 5) )
         );

xyplot(y ~ x, temp, cex = temp$cex, pch = 19);

This works just fine if I create a straight xy-plot, without groups.   
However when I introduce groupings the cex argument specifies the  
point-size for the entire group.  For example:

xyplot(y ~ x, temp, cex = temp$cex, pch = 19, group = groups);

Is it possible to combine per-spot sizing with groups in some way?   
One work-around is to manually specify all graphical parameters, but I  
thought there might be a better way than this:

temp$col <- rep("blue", 10);
temp$col[temp$groups == "B"] <- "red";
xyplot(y ~ x, temp, cex = temp$cex, pch = 19, col = temp$col);

Any suggestions/advice is much appreciated!
Paul