R Graphics Cookbook

Chapter 8. Axes

The x- and y-axes provide context for interpreting the displayed data. Ggplot2 will display the axes with defaults that look good in most cases, but you might want to control, for example, the axis labels, the number and placement of tick marks, the tick mark labels, and so on. In this chapter, I’ll cover how to fine-tune the appearance of the axes.

Swapping X- and Y-Axes

Problem

You want to swap the x- and y-axes on a graph.

Solution

Use coord_flip() to flip the axes (Figure 8-1):

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

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

Figure 8-1. Left: a box plot with regular axes; right: with swapped axes

Discussion

For a scatter plot, it is trivial to change what goes on the vertical axis and what goes on the horizontal axis: just exchange the variables mapped to x and y. But not all the geoms in ggplot2 treat the x- and y-axes equally. For example, box plots summarize the data along the y-axis, the lines in line graphs move in only one direction along the x-axis, error bars have a single x value and a range of y values, and so on. If you’re using these geoms and want them to behave as though the axes are swapped, coord_flip() is what you need.

Sometimes when the axes are swapped, the order of items will be the reverse of what you want. On a graph with standard x- and y-axes, the x items start ...