Add Label to Straight Line in ggplot2 Plot in R (2 Examples)

In this article you’ll learn how to add a labeled line to a ggplot2 graphic in R programming.

The post contains the following contents:

Let’s dive into it.

Example Data, Add-On Packages & Default Graphic

We use the following data as basement for this R programming tutorial:

data <- data.frame(x = 1:5,         # Create example data
                   y = 6:10)
data                                # Print example data

Table 1 illustrates the structure of our example data – It consists of five lines and two integer columns.

In order to use the functions of the ggplot2 package, we also have to install and load ggplot2:

install.packages("ggplot2")         # Install ggplot2 package
library("ggplot2")                  # Load ggplot2

Now, we can plot our data as follows:

ggp <- ggplot(data, aes(x, y)) +    # Create plot without line
  geom_point()
ggp                                 # Draw plot without line

As shown in Figure 1, the previously shown syntax has created a ggplot2 scatterplot without any lines or labels.

Example 1: Labeling a Horizontal Line in a ggplot2 Plot

This example explains how to add a straight horizontal line with a label to our ggplot2 plot.

First, we have to define the location on the y-axis of our straight line:

h_line <- 8.7                       # Position of horizontal line

Next, we can use the geom_hline and geom_text functions to add a straight line with a text label to our ggplot2 graphic:

ggp +                               # Add horizontal line & label
  geom_hline(aes(yintercept = h_line)) +
  geom_text(aes(0, h_line, label = h_line, vjust = - 1))

By running the previous R programming syntax we have created Figure 2, i.e. a ggplot2 scatterplot with straight line and label.

Example 2: Labeling a Vertical Line in a ggplot2 Plot

This section demonstrates how to add a straight vertical line with label to a ggplot2 graph.

As in the previous example, we first have to set the position of our line:

v_line <- 3.3                       # Position of vertical line

Now, we can apply the geom_vline and geom_text functions to add our vertical line with label:

ggp +                               # Add vertical line & label
  geom_vline(aes(xintercept = v_line)) +
  geom_text(aes(v_line, 4, label = v_line, hjust = - 1))

Video & Further Resources

Do you want to know more about lines in ggplot2 graphs? Then you might want to watch the following video of my YouTube channel. In the video, I’m explaining the contents of this post in R.

Furthermore, you could have a look at some of the related articles on this website:

• Add Text to ggplot2 Plot
• Detailed Introduction to the ggplot2 Package
• Add Individual Text to Each Facet of ggplot2 Plot
• Change Colors in ggplot2 Line Plot
• Graphics Overview in R
• Introduction to R Programming

In summary: In this R programming tutorial you have learned how to draw a straight labeled line in a ggplot2 plot. Please let me know in the comments section, in case you have any further questions.