How do I add an asterix to a boxplot to represent significance?

Question

I am trying to include an asterix on top or bottom of a box in boxplot graph that represent significance after performing the independent variable t-test evaluation. How can I add this to my graph?

Answer 1

You can use text() and write at the corresponding location, if you know it beforehand; e.g.,

dfrm <- data.frame(y=rnorm(100, mean=10), x=gl(4, 25))
dfrm$y[dfrm$x==2] <- dfrm$y[dfrm$x==2]+2
boxplot(y ~ x, data=dfrm, ylim=c(min(dfrm$y)-.5, max(dfrm$y)+.5))
text(x=2, y=max(dfrm$y[dfrm$x==2]), "*", pos=3, cex=1.2)

Adapt x=2 to suit your needs.

Or you can use mtext to put the star outside the plotting region, like in

mtext("*", side=3, line=0, at=2, cex=1.2)

Answer 2

You can use text() to add simple symbols to custom location on the plot:

boxplot(c(1:10),ylim=c(0,12),axes=F)
text(11,"*",cex=2)

EDIT: In reply to @chl suggestion, here is the above application with a series of boxplots:

boxplot(count ~ spray,data = InsectSprays,axes=F,ylim=c(0,30))
text(c(25,23,-10,-10,-10,27),"*",cex=2)