[R] again Trellis plot

[Brunschwig, Hadassa {PDMM~Basel}]

Hello everyone

I've got another question on Trellis plots. I plotted my fitted model with plot(augPred(fitted model)). According to the description of this method, one can use any optional argument which is passed down to the xyplot function for Trellis plots. Additionally to the trivial plot i would like to insert a horizontal line. I tried to do it with the argument 
panel= function (...) {panel.abline(vector wich contains a value for each panel to draw a horizontal line on the y axis)}. 
The plot was overridden by this statement and i got panels with horizontal lines ( in each panel the whole vector was drawn).
How can do this differently?
Thanks a lot for answers.

Dassy