Lattice: groups and no groups with panel.superpose

Dear R listers,

I am trying to create a lattice plot with independent regression  
lines per group
for each panel, plus one overall regression line per panel. I  
succeed in having
the separate regression lines for each group per panel, but I can't  
figure out
how to add the overall regression line over all groups. I have  
learnt a lot from
the list, but I can't find an example for what I want to do...

with the following example data (from another post):
 years<-2000:2006
 weeks<-1:20
 yr<-rep(years,rep(length(weeks)*6,length(years)))
 wk<-rep(weeks,rep(6,length(weeks)))
 temp<-rep(4:9,length(years)*length(weeks))
 yvar<-round(rnorm(length(years)*length(weeks)*6,mean=30,sd=4),0)
 xvar<-(rnorm(length(years)*length(weeks)*6)+5)/10
 df<-data.frame(year=yr,week=wk,temp=temp,       yvar=yvar,
 xvar=xvar)

I get the regression lines per group for each panel using:

 xyplot(yvar~year|week,data=df,layout = c(4, 5),
        type='p',
        groups = temp ,
        panel = panel.superpose,
        panel.groups = function(x, y, ..., lty) {
                panel.xyplot(x, y, ..., lty = lty)
                panel.abline(lm(y~x), lty=3, ...)
        })

And obviously the overall regression lines with:

 xyplot(yvar~year|week,data=df,layout = c(4, 5),
        type='p',
        panel = function(x, y, ...) {
        panel.xyplot(x, y, ...)
        panel.abline(lm(y~x))
        })

Any advice on how to "merge" the output from the two above graphs  
would be
greatly appreciated!


Kind regards,

Robert Buitenwerf
Ecologist
SAEON
South Africa