xyplot, groups and colors

	Dear ExpeRts,

I am trying to plot a bunch of growth curves and would like to get
some more control over groups and line colors than I seem to have.

Example:

# make some data
dat <- Orange
dat$group <- ifelse(dat$Tree%in%c('1','4','5'), 'A', 'B')

# plot
xyplot(circumference~age, dat, groups=group)

# now use lines to make the growth curve more visible
xyplot(circumference~age, dat, groups=group, type='l')
# ugly, because of the 'return' lines

# to fix this set groups to Tree
xyplot(circumference~age, dat, groups=Tree, type='l')
# better, but now each Tree has its own color

Of course I can now use the col argument to manually assign the colors
by group but is there a more elegant way that I missed?

xyplot(circumference~age, dat, groups=Tree, type='l',
          col.line = c('red', 'blue', 'blue', 'red', 'red'))

After a little more fiddling around, this also works, and seems a bit less
kludgy:

dat$group2 <- factor(dat$group, labels = c('red', 'blue'))
xyplot(circumference~age, dat, groups=Tree, type='l',
          col.line = levels(dat$group2))