You want to set the range (or limits) of an axis.

You can use xlim() or ylim() to set the minimum and maximum values of a continuous axis. Figure 8-3 shows one graph with the default y limits, and one with manually set y limits:

p <- ggplot(PlantGrowth, aes(x=group, y=weight)) + geom_boxplot()
# Display the basic graph
p

p + ylim(0, max(PlantGrowth$weight))

Figure 8-3. Left: box plot with default range; right: with manually set range

The latter example sets the y range from 0 to the maximum value of the weight column, though a constant value (like 10) could instead be used as the maximum.

ylim() is shorthand for setting the limits with scale_y_continuous(). (The same is true for xlim() and scale_x_continuous().) The following are equivalent: