[R] Adding different output to different lattice panels

[alxsal at free.fr]

I would like to add a reference line to lattice graphs, with the reference line
being different according to the factor level.

Example : Draw 3 dotplots for "a","b" and "c" factors, and then add an
horizontal line at y=10 for panel "a", y=8 for panel "b" and y=6 for panel "4"

I tried the code below, but this draw all three reference lines for each panel.
How do I index the current panel to chose the right reference vector value ?

dat<-data.frame(id=rep(c("a","b","c"),4),val=1:12,quand=rep(c("t1","t2","t3","t4"),each=3))
ref<-c(10,8,6)
plot.new()
datplot<-dotplot(val~quand|id,data=dat,panel=function(...){
panel.dotplot(...)
panel.abline(h=ref)
})
print(datplot)