[R] add arrows to barchart with groups

[GOUACHE David]

Hello Rhelpers,

I am trying to represent the following data (hereafter named donnees) in a barchart with a grouping variable :


site	traitement	date	s	res
1	NT	17/10/2005	normal	76.2
1	T	17/10/2005	normal	103.2
1	NT	23/11/2005	tardif	81.6
1	T	23/11/2005	tardif	98
2	NT	15/10/2005	normal	72.71
2	T	15/10/2005	normal	94.47
2	NT	15/11/2005	tardif	79.65
2	T	15/11/2005	tardif	94.7

barchart(res~s|site,groups=traitement,data=donnees)

What I'd like to do is for each site represent with an arrow the difference in value of variable res between normal and tardif values of variable s.
I've found one way of doing it:


trellis.focus("panel",1,1)
xx<-trellis.panelArgs()$x
yy<-trellis.panelArgs()$y
panel.arrows(as.numeric(xx)[c(1,3)]-0.1,yy[c(1,3)],as.numeric(xx)[c(1,3)]-0.1,yy[c(2,4)],lwd=2,code=3)
panel.text(as.numeric(xx)[c(1,3)]-0.35,c(87,87),paste(yy[c(2,4)]-yy[c(1,3)],"\nq/ha"),font=2)
trellis.focus("panel",2,1)
xx<-trellis.panelArgs()$x
yy<-trellis.panelArgs()$y
panel.arrows(as.numeric(xx)[c(1,3)]-0.1,yy[c(1,3)],as.numeric(xx)[c(1,3)]-0.1,yy[c(2,4)],lwd=2,code=3)
panel.text(as.numeric(xx)[c(1,3)]-0.35,c(87,87),paste(yy[c(2,4)]-yy[c(1,3)],"\nq/ha"),font=2)
trellis.unfocus()

But I would prefer doing this within a custom panel function so I can apply it more generally, and I haven't been able to figure out how...
Could anyone give me a hand?
Thanks in advance.

David Gouache
Arvalis - Institut du Végétal