4 Labels

4.1 Introduction

Why this chart

Axis labels and titles turn a plot into a message; they are the cheapest readability win.

Common pitfall

Tiny or default labels (e.g. snake_case column names) force readers to guess the message.

In this chapter, we will learn to:

add title and subtitle to the plot
modify axis labels
modify axis range
remove axis
format axis

4.2 Basic Plot

Let us start with a simple scatter plot. We will continue to use the mtcars data set and examine the relationship between displacement and miles per gallon using geom_point().

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

4.3 Title & Subtitle

There are two ways to add title to a plot:

ggtitle()
labs()

4.3.1 ggtitle()

Let us explore the ggtitle() function first. It takes two arguments:

label: title of the plot
subtitle: subtitle of the plot

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  ggtitle(label = 'Displacement vs Mileage', subtitle = 'disp vs mpg')

4.4 Axis Labels

You can add labels to the axis using:

xlab()
ylab()
labs()

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  xlab('Displacement') + ylab('Miles Per Gallon')

4.5 Labs

The labs() function can be used to add the following to a plot:

title
subtitle
X axis label
Y axis label

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  labs(title = 'Displacement vs Mileage', subtitle = 'disp vs mpg', 
    x = 'Displacement', y = 'Miles Per Gallon')

4.6 Axis Range

In certain scenarios, you may want to modify the range of the axis. In ggplot2, we can achieve this using:

xlim()
ylim()
expand_limits()
xlim() and ylim() take a numeric vector of length 2 as input
expand_limits() takes two numeric vectors (each of length 2), one for each axis
in all of the above functions, the first element represents the lower limit and the second element represents the upper limit

4.6.1 X Axis

In the below example, we limit the range of the X axis between 0 and 600 using xlim.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  xlim(c(0, 600))

4.6.2 Y Axis

Let us limit the range of the Y axis between 0 and 40.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  ylim(c(0, 40))

4.6.3 Expand Limits

Let us use expand_limits() to limit the range of both the X and Y axis. The first input is the range for the X axis and the second input for the Y axis. In both the cases, we use a numeric vector of length 2 to specify the lower and upper limit.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  expand_limits(x = c(0, 600), y = c(0, 40))

4.6.4 Remove Axis Labels

If you want to remove the axis labels all together, use the theme() function. It allows us to modify every aspect of the theme of the plot. Within theme(), set the following to element_blank().

axis.title.x
axis.title.y

element_blank() will remove the title of the X and Y axis.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) +
  theme(axis.title.x = element_blank(), axis.title.y = element_blank())

4.7 Format Title & Axis Labels

To format the title or the axis labels, we have to modify the theme of the plot using the theme() function. We can modify:

color
font family
font face
font size
horizontal alignment
and angle

In addition to theme(), we will also use element_text(). It should be used whenever you want to modify the appearance of any text element of your plot.

4.7.1 Color

In the below example, we use the color argument within element_text() to modify the color of the title of the plot to 'blue'.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) + ggtitle('Diaplacement vs Mileage') +
  theme(plot.title = element_text(color = 'blue'))

4.7.2 Font Family

Let us change the font family of the plot title to 'Arial' by using the family argument.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) + ggtitle('Diaplacement vs Mileage') +
  theme(plot.title = element_text(family = 'Arial'))

4.7.3 Font Face

The font face can be any of the following:

plain
bold
italic
bold.italic

The face argument can be used to modify the font face of the title of the plot.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) + ggtitle('Diaplacement vs Mileage') +
  theme(plot.title = element_text(face = 'bold'))

4.7.4 Font Size

The size of the title of the plot can be modified using the size argument.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) + ggtitle('Diaplacement vs Mileage') +
  theme(plot.title = element_text(size = 8))

4.8 Horizontal Alignment

To modify the horizontal alignment of the title, use the hjust argument. It can take values between 0 and 1. If the value is closer to 0, the text will be left-aligned and viceversa.

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) + ggtitle('Diaplacement vs Mileage') +
  theme(plot.title = element_text(hjust = 0.3))

4.9 Putting it all together

4.9.1 Title

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) + ggtitle('Diaplacement vs Mileage') +
  theme(plot.title = element_text(color = 'blue', family = 'Arial',
    face = 'bold', size = 12, hjust = 0.5))

4.9.2 X axis label

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) + xlab('Diaplacement') +
  theme(axis.title.x = element_text(color = 'blue', family = 'Arial',
    face = 'bold', size = 8, hjust = 0.5, angle = 15))

4.9.3 Y axis label

ggplot(mtcars) +
  geom_point(aes(disp, mpg)) + ylab('Miles Per Gallon') +
  theme(axis.title.y = element_text(color = 'blue', family = 'Arial',
    face = 'italic', size = 8, vjust = 0.3, angle = 90))

4.10 Exercises

Add x, y and title labels to a disp vs mpg scatter plot.
Rename a legend title with the name argument of a scale function.
Rewrite a default-titled plot so the title states the conclusion.

Worked solutions in solutions/labels.md (attempt first).

Where to go next

ggplot2 book (ggplot2-book.org) for theory behind this layer
Claus Wilke, Fundamentals of Data Visualization, for perception guidance