[R] groups in lattice

[Ruud H. Koning]

Hello, I must be missing something very obvious. I have a dataset with 
consists of three groups, and for each group I would like to draw a 
regression line in one frame. Something like

plot.a <- xyplot(result~price,data=nl,
 xlab="Standardized price",ylab="Outcome",groups=slice,
 panel=function(x,y,...){
  panel.lmline(x,y,...)
 }
)

However, only one line is drawn. If I replace panel.lmline by 
panel.densityplot(x,...), I do get three densityplots in the frame. What 
am I missing?
Thanks, Ruud