adding horizontal "separating" lines in a single boxplot in Base R plotting

Question

I need to add a "separating" line in Base R boxplot to separate difference groups. In the example below, I want to separate groups A and B (each having 2 levels) using a horizontal line (in red). R codes for reproducible results:

dat = data.frame(A1 = rnorm(1000, 0, 1), A2 = rnorm(1000, 1, 2),
                 B1 = rnorm(1000, 0.5, 0.5), B2 = rnorm(1000, 1.5, 1.5))
boxplot(dat, horizontal = T, outline=F)

Is there an easy way to do in Base R?

Also, is there an easy way to color the y-axis labels? I want to have A1 and B1 shown as red, and A2 and B2 shown as blue in the axis.

Thanks!

Answer 1

Use abline. To get the right position take the mean of the axTicks of the y-axis.

To get the colored labels, first omit yaxt and rebuild axis ticks and mtext, also using axTicks.

b <- boxplot(dat, horizontal=T, outline=F, yaxt="n")
ats <- axTicks(2)
axis(2, labels=F)
mtext(b$names, 2, 1, col=c(2, 4), at=ats)
abline(h=mean(ats), lwd=2, col=2)

If you want axis tick label colors corresponding to the labels, use segments instead.

b <- boxplot(dat, horizontal=T, outline=F, yaxt="n")
ats <- axTicks(2)
abline(h=mean(ats), lwd=2, col=2)
pu <- par()$usr
Map(function(x, y) segments(pu[1] - .2, x, pu[1], x, xpd=T, col=y), ats, c(2, 4))
mtext(b$names, 2, 1, col=c(2, 4), at=ats)

Edit: To adjust the space a little more use at=option in boxplot and leave out the middle axTicks.

b <- boxplot(dat, horizontal=T, outline=F, yaxt="n", at=c(1, 2, 4, 5))
ats <- axTicks(2)[-3]
abline(h=mean(ats), lwd=2, col=2)
pu <- par()$usr
Map(function(x, y) segments(pu[1] - .2, x, pu[1], x, xpd=T, col=y), ats, c(2, 4))
mtext(b$names, 2, 1, col=c(2, 4), at=ats)