The end of the line for error bars in R

When plotting in R, I often use the segments function to add lines representing confidence intervals. This is a very simple way to plot lines connecting pairs of x,y coordinates.

Recently I discovered that by default, segments are styled with rounded line caps, which add to their length. This means, of course, that confidence intervals are slightly wider than intended.

R provides three styles of line ending – round, butt and square – which can be specified by the lend argument. The figure here shows the outcome of using each line ending, with vertical lines indicating actual end-points of segments. Both round and square line ends overshoot these points, while butt ends represent them correctly.

plot.new()
par(mar=c(1, 4, 1, 1))
plot.window(xlim=c(0, 1), ylim=c(0.5, 3.5))
axis(2, 1:3, c('round', 'butt', 'square'), las=1)
box(lwd=2)
segments(0.1, 1, 0.9, 1, lwd=20, lend='round')
segments(0.1, 2, 0.9, 2, lwd=20, lend='butt')
segments(0.1, 3, 0.9, 3, lwd=20, lend='square')
abline(v=c(0.1, 0.9))

Line end styles applied to segments plotted in R. Only ‘butt’ accurately represents end points.

The effect is slight, and is emphasized when line width is large. Regardless, it’s a good idea to routinely add lend='butt' (or lend=2) to your segments function calls.

A secondary benefit is that lines will appear crisper than when plotted with the default round caps.