[R] change plotting symbol for groups in trellis graph

[Tom Boonen]

Hi,

how can I change the plotting symbol for the groups in a trellis panel dotplot.

My graph is similar to:

library(trellis)
dotplot(variety ~ yield | site, data = barley, groups = year,
             key = simpleKey(levels(barley$year), space = "right"),
             xlab = "Barley Yield (bushels/acre) ",
             aspect=0.5, layout = c(1,6), ylab=NULL)

I'd like to plot the different years with different symbols (eg. one
with pch=19 the other with pch=21 or so). Thank you very much for your
help.

Tom