adding a reference line to an xyplot

Hi,

I want to add a dashed vertical line to a number of xyplots.

Here is a simple script of the type of plot I have but then I want to add a
reference line to 1995 on each of the panels.  I have tried panel.abline and
other suggestions on the forum but can't get it to work.

plot<-rep(letters[1:3],each=10)
year<-rep(1991:2000,times=3)
matter<-rep(seq(2,6.5,0.5),times=3)
xyplot(matter~year|plot,type="l")

Cheers,
Chris

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