[R] Different gridlines per panel in xyplot

[Rene Braeckman]

In the example R script below, horizontal gray gridlines are drawn at y
coordinates where the points are drawn with the code:
 
panel.abline(h=y, v=xScale, col.line="gray")

How do I change this so that the horizontal gray gridlines are drawn at y
coordinates where the y labels are drawn? The challenge is that each panel
has different y-ranges (in my real example the y-ranges and y-intervals are
even more different). For example, I wish I could use the yScale list as the
h parameter in abline, but it does not work with a list.
 
Thanks for any help.
Rene
 
library(lattice)
Subj <- rep(1:4,each=3)
Time <- rep(1:3,4) + 0.1
Conc <- (1:12) + 0.1
df <- data.frame(Subj,Time,Conc)
xScale <- 1:3
yScale <- list(1:3,4:6,7:9,10:12)
xyplot(Conc ~ Time | Subj,
       data = df,
       layout = c(2,2),
       type="b",
       scales=list(
          x=list(at=xScale),
          y=list(at=yScale,relation="free")
       ),
       panel = function(x,y,...) {
           panel.abline(h=y, v=xScale, col.line="gray")
           panel.xyplot(x,y,...)
      }
)