[R] plotting grouped data object

[Afshartous, David]

All,
I'd like to plot the main relationship of a grouped data 
object for all levels of a factor in a single panel.
The sample code below creates a separate panel for each level
of the factor.  I realize that this could be done in other ways, 
but I'd like to do it via plotting the grouped data object.
thanks!
dave

z = rnorm(18, mean=0, sd=1)
x = rep(1:6, 3)
y = factor(rep(c("I", "C", "P"), each = 6))
dat = data.frame(x, y, z)
data.grp = groupedData(z ~ x | y, data = dat)
plot(data.grp, outer = ~ y)
### this produces 1 line each in 3 panels
### how to collapse all 3 lines into 1 panel?