ggplot2: Guides - Axes

Introduction

This is the twelfth post in the series Elegant Data Visualization with ggplot2. In the previous post, we learnt to build histograms. Now that we have learnt to build different plots, let us look at different ways to modify the axis. Along the way, we will also explore the scale_*() family of functions.

Modify X and Y axis

• title
• labels
• limits
• breaks
• position

In this module, we will learn how to modify the X and Y axis using the following functions:

• Continuous Axis
  • scale_x_continuous()
  • scale_y_continuous()
• Discrete Axis
  • scale_x_discrete()
  • scale_y_discrete()

Libraries, Code & Data

We will use the following libraries in this post:

• readr
• ggplot2

All the data sets used in this post can be found here and code can be downloaded from here.

Continuous Axis

If the X and Y axis represent continuous data, we can use scale_x_continuous() and scale_y_continuous() to modify the axis. They take the following arguments:

• name
• limits
• breaks
• labels
• position

Let us continue with the scatter plot we have used in previous posts.

ggplot(mtcars) +
  geom_point(aes(disp, mpg))

The name argument is used to modify the X axis label. In the below example, we change the X axis label to 'Displacement'. In previous posts, we have used xlab() to work with the X axis label.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  scale_x_continuous(name = "Displacement")

To modify the range, use the limits argument. It takes a vector of length 2 i.e. 2 values, the lower and upper limit of the range. It is an alternative for xlim().

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  scale_x_continuous(limits = c(0, 600))

In the above plot, the ticks on the X axis appear at 0, 200, 400 and 600. Let us say we want the ticks to appear more closer i.e. the difference between the tick should be reduced by 50. The breaks argument will allow us to specify where the ticks appear. It takes a numeric vector equal to the length of the number of ticks.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  scale_x_continuous(breaks = c(150, 300, 450))

We can change the tick labels using the labels argument. In the below example, we use words instead of numbers. When adding labels, we need to ensure that the length of the breaks and labels are same.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  scale_x_continuous(breaks = c(150, 300, 450),
    labels = c('One Hundred Fifty', 'Three Hundred', 'Four Hundred Fifity'))

The position of the axes can be changed using the position argument. In the below example, we can move the axes to the top of the plot by supplying the value 'top'.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  scale_x_continuous(position = 'top')

Putting it all together..

ggplot(mtcars) + geom_point(aes(disp, mpg)) +
  scale_x_continuous(name = "Displacement", limits = c(0, 600),
                     breaks = c(0, 150, 300, 450, 600), position = 'top',
                     labels = c('0', '150', '300', '450', '600'))

Y Axis - Continuous

ggplot(mtcars) + geom_point(aes(disp, mpg)) +
  scale_y_continuous(name = "Miles Per Gallon", limits = c(0, 45),
                     breaks = c(0, 15, 30, 45), position = 'right',
                     labels = c('0', '15', '30', '45'))

Discrete Axis

If the X and Y axis represent discrete or categorical data, scale_x_discrete() and scale_y_discrete() can be used to modify them. They take the following arguments:

• name
• labels
• breaks
• position

The above options serve the same purpose as in the case of continuous scales.

ggplot(mtcars) +
  geom_bar(aes(factor(cyl))) +
  scale_x_discrete(name = "Number of Cylinders")

ggplot(mtcars) +
  geom_bar(aes(factor(cyl))) +
  scale_x_discrete(labels = c("4" = "Four", "6" = "Six", "8" = "Eight"))

ggplot(mtcars) +
  geom_bar(aes(factor(cyl))) +
  scale_x_discrete(breaks = c("4", "6", "8"))

ggplot(mtcars) +
  geom_bar(aes(factor(cyl))) +
  scale_x_discrete(position = 'bottom')

ggplot(mtcars) + geom_bar(aes(factor(cyl))) +
  scale_x_discrete(name = "Number of Cylinders",
    labels = c("4" = "Four", "6" = "Six", "8" = "Eight"),
    breaks = c("4", "6", "8"), position = "bottom")

Summary

In this post, we learnt to modify

• title
• labels
• limits
• breaks
• position

Up Next..

In the next post, we will learn to modify the legend when color is mapped to a variable.