You want to change the order of items on a categorical axis.

For a categorical (or discrete) axis—one with a factor mapped to it—the order of items can be changed by setting limits in scale_x_discrete() or scale_y_discrete().

To manually set the order of items on the axis, specify limits with a vector of the levels in the desired order. You can also omit items with this vector, as shown in Figure 8-8:

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

p + scale_x_discrete(limits=c("trt1","ctrl","trt2"))

Figure 8-8. Left: box plot with manually specified items on the x-axis; right: with only two items