Axes - R Graphics - page 224

Solution
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
Discussion
You can also use this method to display a subset of the items on the axis. This will show only
ctrl and trt1 ( Figure 8-8 , right):
p + scale_x_discrete(limits = c( "ctrl" , "trt1" ))
To reverse the order, set limits=rev(levels(...)) , and put the factor inside. This will re-
verse the order of the PlantGrowth$group factor, as shown in Figure 8-9 :
p + scale_x_discrete(limits = rev(levels(PlantGrowth$group)))