[R] lattice and several groups

[Laurent Rhelp]

Dear R-list,

     I would like to use the lattice library to show several groups on 
the same graph. Here's my example :

## the data
f1 <- factor(c("mod1","mod2","mod3"),levels=c("mod1","mod2","mod3"))
f1 <- rep(f1,3)
f2 <- factor(rep(c("g1","g2","g3"),each=3),levels=c("g1","g2","g3"))
df <- data.frame(val=c(4,3,2,5,4,3,6,5,4), x=rep(c(1,2,3),3),f1=f1,f2=f2)
#############################################################
library(lattice)

para.liste <- trellis.par.get()
superpose.symbol <- para.liste$superpose.symbol
superpose.symbol$pch <- c(1,2,3)
trellis.par.set("superpose.symbol",superpose.symbol)

# Now I can see the group according to the f1 factor (with a different 
symbol for every modality)
xyplot( val~x,   
        data=df,
        group=f1,
        auto.key=list(space="right")
       )

# or I can see the group according to the f2 factor
xyplot( val~x,   
        data=df,
        type="l",
        group=f2,
        auto.key=list(space="right",points=FALSE,lines=TRUE)
       )

How can I do to highlight both the f1 and f2 factors on one panel with 
the legends, using the lattice function ?

Thanks