[R] Lattice: How to do error bars

[Alex van der Spek]

I am trying to add 2 stdev error bars to lattice type plots:

panel.ebar<-function(x,y,dy=NULL,...) {
	panel.xyplot(x,y,...)
	panel.segments(x,y-dy,x,y+dy,...)
}

Then:

xyplot(y~x|fc,data=dat,dy=dat$dy,panel=panel.ebar)

This adds error bars but they are not conditioned on the factor fc.

xyplot(y+I(y-dy)+I(y+dy)~x|fc,data=dat)

This produces 3 series of points in different colors, conditioned on fc
as the data is now grouped per panel. 

Obviously I need a combination of the above two. I can't figure it out.

Any help? Just point to docs if avaiable. The R doc is excellent. Most
of the time I can save myself.

Thanks in advance!
Alex van der Spek