i appreciate all the interest in my question, and thank you Elai for both of your suggestions, which work very well. Elai, your last code was particularly simple and helpful to generate the figure i was looking for.
x11(height=8,width=11) par(lend=2) xdat = data.frame(mortality =c(5, 8,7,5,8,10,11,6,4,5,20,25,27,30,35,32,28,21,20,34,11,15,18,12,15,12,10,15,19,20), type= c(1, 1,1,1,1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,3,3,3,3,3,3,3,3,3,3), attend = c(1, 0,1,1,1,1,0,0,0,1,0,1,0,0,0,0,1,0,0,0,1,1,1,0,1,0,1,0,0,0)) xyplot ( mortality ~ factor(attend)|type, panel=function(x,y) { panel.grid(lty=5) panel.xyplot(x,y,pch=16,jitter.x=TRUE,col=1) yy<- tapply(y,x,median) panel.segments(x0=c(0.7,1.7), x1=c(1.3,2.3), y0= yy , y1= yy , col="red",lwd=5) }, data = xdat, aspect=2:1,layout=c(3,1)) http://r.789695.n4.nabble.com/file/n4541912/trial.jpg as a side note, the par(lend=2) function did not square the ends of the lines for panel.segments in xyplot (i was trying to get square ends instead of round ends). John: thank you for your information regarding latticeExtra and the layer function, which will be helpful for future figures. -- View this message in context: http://r.789695.n4.nabble.com/Drawing-a-line-in-xyplot-tp4538689p4541912.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.