R help - May 2011 - Draftsman plot axis modification

Dear all,

I want to draw a draftsman plot using the pairs function and omit the axis
labels of the diagonal panel which does not work.
Is it also possible to remove the boxes of the diagonal panel?

Here is an example what the graph looks like

##################################
PC1<-c(1.1,1.2,1.9,2.0,2.5,2.6)
PC2<-PC1+2
PC3<-PC2+4
group<-factor(c("A","A","B","B","C","C"))

dat<-data.frame(group,PC1,PC2,PC3)

pairs(dat[2:4],upper.panel=NULL,pch=21,
bg=c("red","blue","green")[unclass(dat$group)],
cex=2)
##############################

I would appreciate to receive any tips to solve my problem.

Thanks in advance,

Michael
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