[R] Double x grid in ggplot2

[James Rome]

I am trying to overlay raw data with a boxplot as follows:
        pp = qplot(factor(time, levels=0:60, ordered=TRUE),
            error, data=dfsub, size=I(1), main =" title", ylab="Error
(min)",
            xlab="Time before ON (min)", alpha=I(1/10),
ylim=c(-30,40),    geom="jitter") +
            facet_wrap(~ runway, ncol=2) +
            geom_boxplot(alpha=.5, color="blue", outlier.colour="green",
outlier.size=1) +
            scale_x_discrete(breaks = seq(from=0, to=60, by=10))
        print(pp)

But I think ggplot2 is getting confused about factors versus numbers
> sapply(dfsub,class)
     time     error    runway    flight
"numeric" "numeric"  "factor"  "factor"
because when I do the plot, the vertical grid lines are in pairs, one
line at (0, 10, 20,...) and the other at (-1, 9, 19,....)

 If I remove the scale_x_discrete, I get a grid line at every minute,
and things look right, but the labels for the minutes all overlap.

Thanks for the help,
Jim Rome