[R] Overlaying lattice graphs

[Seb]

Hello

I apologize in advance if this question has already be posted on the 
list, although I could not find a relevant thread in the archives.

I would like to overlay xyplots using different datasets for each plot. 
I typically work on the following data.frame (mydata) structure

>mydata
        Drug    Time        Observed          Predicted
1       A        0.05         10                 10.2
2       A        0.10         20                 19.5
etc...
100     B        0.05         11                 12.7
101     B        0.10         35                 36
etc...

I want to plot the observed data as points and the predicted values as 
lines. If I use the following commands, I don't have the possibility to 
switch the "y" values from Observed for the scatterplot to Predicted for 
the line.

xyplot(Observed ~ Time | Drug, data = mydata, panel  =  function(x,y, ...){
+            panel.xyplot(x,y,...)
+            panel.xyplot(x,y,type="l",...)})

I wonder if this problem can be solved using the trellis.focus "family" 
commands but I have a hard time to understand how they work.

Please, let me know if a thread have already addressed this question. 
Otherwise, I would grateful for any hint, comments or info you can provide.

Thanks

Sebastien