[R] xyplot plot with groups

[array chip]

Hi, I am a beginner of xyplot() (or lattice package). On one hand, I immediately 
realized it's a very powerful utility. On the other hand, there are too many 
things for me to learn. Still haven't figure out a generalization of the syntax 
and usage under many different circumstances.

Let me give an example dataset:


xxx<-cbind(expand.grid(day=0:7,trt=c('A','B','C'),sex=c('M','F')),subject=rep(1:60,each=8))

xxx<-xxx[sort.list(xxx$trt),]
y<-c(rnorm(160,15),rnorm(160,10),rnorm(160,5))

dat<-cbind(y=y,xxx)

str(dat)
'data.frame':   480 obs. of  5 variables:
 $ y      : num  13.4 15.3 14.4 16.2 15.1 ...
 $ day    : int  0 1 2 3 4 5 6 7 0 1 ...
 $ trt    : Factor w/ 3 levels "A","B","C": 1 1 1 1 1 1 1 1 1 1 ...
 $ sex    : Factor w/ 2 levels "M","F": 1 1 1 1 1 1 1 1 2 2 ...
 $ subject: int  1 1 1 1 1 1 1 1 4 4 ...

So this is a typical longitudinal dataset: each subject has measurements for 8 
days (0 to 7), there are 3 treatments (A, B and C), and sex factor (M and F). I 
would like to have 2 plots, one for each sex. Within each plot, I want to plot 3 
averaged lines (average of y for each day), one for each treatment group. How 
can I do this?

I tried the following using panel.average(), but it seems that the groups 
information was not passed automatically to panel.average(). 


xyplot(y~day|sex, groups=trt, data=dat, type=c('p','g'), panel=function(...) 
{panel.xyplot(...); panel.average(..., fun=function(x) 
mean(x,na.rm=T),horizontal=F,col=1:3)})

Appreciate if anyone can help.

John