Modify axis size when using gap.plot

I am creating a plot with axes breaks using the package plotrix, but the standard cex.axis does not seem to work when I build a gap.plot.

Do you know how to proceed to modify the size of the axis?

If you set cex.axis using the par command, you can modify the size of the magnification for axis annotation:

library(plotrix)

twogrp <- c(rnorm(5)+4,rnorm(5)+20,rnorm(5)+5,rnorm(5)+22)
gpcol <- c(2,2,2,2,2,3,3,3,3,3,4,4,4,4,4,5,5,5,5,5)

par(cex.axis=2, cex.lab=1.5)
gap.plot(twogrp, gap=c(8,16), xlab="Index", ylab="Group values",
  main="Gap on Y axis", col=gpcol)