You want to set where the tick marks appear on the axis.

Usually ggplot() does a good job of deciding where to put the tick marks, but if you want to change them, set breaks in the scale (Figure 8-12):

ggplot(PlantGrowth, aes(x=group, y=weight)) + geom_boxplot()

ggplot(PlantGrowth, aes(x=group, y=weight)) + geom_boxplot() +
    scale_y_continuous(breaks=c(4, 4.25, 4.5, 5, 6, 8))

Figure 8-12. Left: box plot with automatic tick marks; right: with manually set tick marks

The location of the tick marks defines where major grid lines are drawn. If the axis represents a continuous variable, minor grid lines, which are fainter and unlabeled, will by default be drawn halfway between each major grid line.