lattice xyplot: plot multiple lines with different colors

Hi everyone,

I am using the lattice package to plot some simulation results, by using 
the function xyplot(). However, I cannot find a way to plot multiple 
lines within the same xyplot and to have each of the lines be drawn in a 
different color.

This is what I am currently doing:

xyplot(a + b + c ~ x, my_data, panel = panel.lines)

but, of course, all lines are drawn in the same color.


What I would prefer, would be to first print a single line in an xyplot 
and then add a new line, and so on (since I want to dynamically 
determine the number of lines that need to be plotted). For example, 
something like this would be nice:

for(i = 1:length(lines_to_be_drawn)) {
     xyplot(lines_to_be_drawn[i] ~ x, my_data, panel = {some function 
setting a different color});

	// do some other functions like plotting legend, parameters
}

(although in this putative example, a new xyplot is drawn over the old 
one every time).

anyone any idea to plot multiple lines with different colors in a single 
xyplot? I'd like to stick to the lattice package, since this package has 
more layout possibilities over the traditional plotting functions in R.

cheers,
Bram