A second layer in the plot we wish to make involves adding a label to each point to identify the state. The geom_label and geom_text functions permit us to add text to the plot with and without a rectangle behind the text, respectively.

Because each point (each state in this case) has a label, we need an aesthetic mapping to make the connection between points and labels. By reading the help file, we learn that we supply the mapping between point and label through the label argument of aes. So the code looks like this:

p + geom_point(aes(population/10^6, total)) +
  geom_text(aes(population/10^6, total, label = abb))

We have successfully added a second layer to the plot.

As an example of the unique behavior of aes mentioned above, note that this call:

p_test <- p + geom_text(aes(population/10^6, total, label = abb))

is fine, whereas this call:

p_test <- p + geom_text(aes(population/10^6, total), label = abb) 

will give you an error since abb is not found because it is outside of the aes function. The layer geom_text does not know where to find abb since it is a column name and not a global variable.

Tinkering with arguments

Each geometry function has many arguments other than aes and data. They tend to be specific to the function. For example, in the plot we wish to make, the points are larger than the default size. In the help file we see that size is an aesthetic and we can change it like this:

p + geom_point(aes(population/10^6, total), size = 3) +
  geom_text(aes(population/10^6, total, label = abb))

size is not a mapping: whereas mappings use data from specific observations and need to be inside aes(), operations we want to affect all the points the same way do not need to be included inside aes.

Now because the points are larger it is hard to see the labels. If we read the help file for geom_text, we see the nudge_x argument, which moves the text slightly to the right or to the left:

p + geom_point(aes(population/10^6, total), size = 3) +
  geom_text(aes(population/10^6, total, label = abb), nudge_x = 1.5)

This is preferred as it makes it easier to read the text. In Section 7.11¹ we learn a better way of assuring we can see the points and the labels.