Dear all,
   
  I have created  the following data (that you can run) in order to  explain my problem:
   
  y <- rep(c(1,2), 8)
  id <- rep(1:8,each=2)
x1 <- rep(c("-a","+a"), each = 8)
x2 <- rep(c("-b","+b"), each = 2, times = 4)
x3 <- rep(c("-c", "+c"), each = 4,2)
df <- data.frame(cbind(id,y,x1,x2,x3))
   
  If I do:
   
  xyplot(y~ x3|x1*x2,data=df,groups=id,type = "b")
   
  then my id's  are joined by lines which is what I want. However when I wanted to add an horizontal line for each panel by using "panel.abline" the lines disapears, i.e.
   
  xyplot(y~ x3|x1*x2,data=df,groups=id,type = "b",panel=function(x,y)
{
panel.xyplot(x,y)
panel.abline(h=1.5)
}
) 
   
   
  I would be grateful if someone can tell me how can I correct the second statement in order to have horizontal lines for each panel and each id are joined  by lines.
   
  Thank you,
   
  Bernard,
   
   

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