[R] adding to Trellis plots

[Brunschwig, Hadassa {PDMM~Basel}]

Hello everyone

I have data grouped by subjects and i used the trellis plot to show the curve for each subject. I have another vector "a" which gives me the highest points the curves for each subjects can achieve ( the curve is exponential ). Now i want to draw a horizontal line, whose values are saved in "a", on each plot for each subject. Is there any possibility to do that? Obviously i should use something like panel.abline, the problem is how i can inform R that each value in "a" is a horizontal line for each plot.

Thanks a lot for answers.

Dassy