Not exactly sure what you want to do, but ...
In your example, you do not need groups, since the color doesn't change
within the levels of the conditioning variable (fact). Hence you can use the
panel.number() function to choose the plotting color of each panel, like
this:
## .. continuing with your example
myColors <- rep(c(2,4),2)
xyplot(y ~ x | fact, data = my.df,
panel = function(...){
panel.xyplot(...,col=myColors[panel.number()])
}
)
If you actually **need** groups (to color different subsets of the data
within a panel differently). it does get a bit more complicated.
Incidentally, note that col is already a formal argument of panel.superpose
and was therefore picked up in the ... argument of the panel.groups
function -- that's why you got the error you did when you repeated col
explicitly in the panel.xyplot call. By default, it's values are those of
trellis.par.get("superpose.symbol") I believe. Also, group.number appears to
be undefined in your code.
HTH
Cheers,
Bert Gunter
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