Draw Plot with Two Y-Axes in R (Example)

In this R programming tutorial you’ll learn how to create a plot with two y-axes in Base R.

Table of contents:

• Creation of Example Data
• Example: Create Plot with 2 Axes in R
• Video, Further Resources & Summary

Here’s how to do it:

Creation of Example Data

First, we need to create some example data for the plot of this example:

set.seed(25852)                             # Create example data
x <- rnorm(30)
y1 <- x + rnorm(30)
y2 <- x + rnorm(30, 5)

Our example data consists of three numeric vectors: x, y1, and y2.

Now, let’s plot these data!

Example: Create Plot with 2 Axes in R

If we want to draw a plot with two different y-axes, we can use the following R code:

par(mar = c(5, 4, 4, 4) + 0.3)              # Additional space for second y-axis
plot(x, y1, pch = 16, col = 2)              # Create first plot
par(new = TRUE)                             # Add new plot
plot(x, y2, pch = 17, col = 3,              # Create second plot without axes
     axes = FALSE, xlab = "", ylab = "")
axis(side = 4, at = pretty(range(y2)))      # Add second axis
mtext("y2", side = 4, line = 3)             # Add second axis label

Figure 1: Plot with 2 Y-Axes in R.

Figure 1 is illustrating the output of the previous R syntax. As you can see, we created a scatterplot with two different colors and different y-axis values on the left and right side of the plot.

How did we do that? Let’s deconstruct the code:

1. par(mar = c(5, 4, 4, 4) + 0.3) – This code defines how much white space should be shown around the plot. It is important to leave enough space for the second y-axis.
2. plot(x, y1, pch = 16, col = 2) – This code creates the first plot (i.e. the red dots).
3. par(new = TRUE) – This code specifies that we want to overlay a second plot.
4. plot(x, y2, pch = 17, col = 3, axes = FALSE, xlab = “”, ylab = “”) – This code draws the second plot (i.e. the green triangles).
5. axis(side = 4, at = pretty(range(y2))) – This code adds the axis labels on the right side.
6. mtext(“y2”, side = 4, line = 3) – This code adds the name of the second y-axis (i.e. y2).

Note that this code is adapted from this thread on Stack Overflow. Have a look at this thread for more examples of the plotting of multiple axes.

Also note that we used a scatterplot for this example. However, you may add a secondary axis to many different types of graphics such as boxplots, line charts, histograms, density plots, and so on…

Video, Further Resources & Summary

I have recently released a video on my YouTube channel, which explains the R programming code of this tutorial. You can find the video below:

Furthermore, you might want to have a look at the related tutorials on my homepage:

• Remove Axis Values of Plot in Base R
• Rotate Axis Labels of Base R Plot
• R Graphics Gallery
• The R Programming Language

In summary: This article explained how to add dual axes to a plot in the R programming language. Don’t hesitate to let me know in the comments, in case you have further comments or questions.