xyplot sorted by date

With the lattice package, I would like to plot C1 as function of C2 sorted
by the dates, that is something like :
xyplot(C1 ~ C2 | as.Date(C3))

It plots somethings but not the truth.
I have compared the output with the output of this script

  

I don't know whether it is ther perfect way but I did something similiar
with using a factor. So you could use factors from 1:(number of days)
and the dates as factor labels. But maybe there is a better way.


hth
Stefan

ps I would be interested in "truth" plotting if someone has a function
it could substitute parts of my reserach ;-)