[R] How to suppress factor labels

[James Rome]

I am using ggplot2 to make a boxplot that overlays a scatterplot:
pp = qplot(time, error, data=times, size=I(1), geom="jitter", main=title,
        ylab="Error (min)", xlab="Time before ON (min)", alpha=I(1/10),
color=times$runway,
        ylim=c(-30,40))
    pp2 = pp + with(times, facet_wrap(~ runway, ncol=2))
    print(pp2 + geom_boxplot(alpha=.5, color="blue",
outlier.colour="green", outlier.size=1))
The x variable is a factor for every minute from 0:60. My problem is
that ggplot2 labels every factor value on the x axis, and they overlap.
How can I make ggplot2 label only, say every 5th factor value on the x axis?

Thanks,
Jim