xyplot problem

Hi all,

I am trying to produce some panels with dots in an X/Y plane where the
diameter of the dots indicates a Z value (like e.g. earthquake maps where dot
sizes indicate magnitudes and X/Y the location).

This works fine with xyplot, e.g.:

xyplot(1:3~1:3,cex=1:3,pch=16)

However, when I do this with a panel variable, e.g.:

x<-rep(1:3,5)
y <- rep(1:3,5)
sz <- rep(1:5,each=3)
grp <- factor(rep(1:5,each=3))
xyplot(y~x | grp , cex=sz)

then sz in the cex argument is not applied per group as I would expect. Same
for other arguments like col.

Is this really the intended behaviour?

How can I achieve what I want, i.e., that the cex argument is different for
the different levels of grp?

Thank you for your help

Pascal Niklaus