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I'm trying to get a plot with wide lines that don't have rounded end caps. You can see the effect quite clearly with a plot like this: plot(0:1) lines(c(1.5,1.5),c(0,.5),lwd=20,col="red") lines(c(1.5,1.5),c(.5,1),lwd=20,col="green") abline(h=0) abline(h=0.5) abline(h=1) The colored lines should be between the horizontal lines, but because of the rounded end caps,