You want to reverse the direction of a continuous axis.

Use scale_y_reverse or scale_x_reverse (Figure 8-6). The direction of an axis can also be reversed by specifying the limits in reversed order, with the maximum first, then the minimum:

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

# Similar effect by specifying limits in reversed order
ggplot(PlantGrowth, aes(x=group, y=weight)) + geom_boxplot() + ylim(6.5, 3.5)

Figure 8-6. Box plot with reversed y-axis

Like scale_y_continuous(), scale_y_reverse() does not work with ylim. (The same is true for the x-axis properties.) If you want to reverse an axis and set its range, you must do it within the scale_y_reverse() statement, by setting the limits in reversed order (Figure 8-7):