[R] lattice: two grouping variables, one controls 'col', the other 'pch'

[Benjamin Tyner]

Say I have

library(lattice)

x<-runif(256)
y<-runif(256)
f<-gl(16,16)
g1<-rep(1:4,each=64)
g2<-rep(1:4,times=64)

plot<-xyplot(y~x|f,
             groups=g1,
             pch=as.character(1:4),
             panel=function(x,y,subscripts,groups,...){
                   panel.superpose(x,y,subscripts,groups,...)
             })            
            
print(plot)

Currently, both color and plotting symbol change with the grouping 
variable g1. What is the best way to have color change with g1, but 
plotting symbol change with g2 (or vice versa)?

Thanks,
Ben