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:
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:
- 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.
- plot(x, y1, pch = 16, col = 2) – This code creates the first plot (i.e. the red dots).
- par(new = TRUE) – This code specifies that we want to overlay a second plot.
- plot(x, y2, pch = 17, col = 3, axes = FALSE, xlab = “”, ylab = “”) – This code draws the second plot (i.e. the green triangles).
- axis(side = 4, at = pretty(range(y2))) – This code adds the axis labels on the right side.
- 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:
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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
- The par Function in R
- 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.
I’m Joachim Schork. On this website, I provide statistics tutorials as well as code in Python and R programming.
12 Comments. Leave new
Great code! Thank you! I am having trouble trying to add a legend to this plot since there are two separate plots layered on one another. Would you be able to show how to add a legend to this plot structure?
Hey Morgan,
Thank you for the nice comment!
You would have to add the legend at the very end of the code. For example, try to add the following code at the end:
Regards
Joachim
Hi, nice code, How I can add space to not superpose the dot. Thanks
Hey Maria,
You may decrease the size of the points and triangles using the cex argument. For instance:
Regards
Joachim
I couldnt solve adding cex = 0.5 because I am doing a boxplot graph, this is my code:
par(mar = c(5, 4, 4, 4) + 0.5)
boxplot (MOR~ sample1, my_data, pch = 16, col = 2, cex=0.5)
par(new = TRUE)
boxplot(density~sample1, my_data, axes= FALSE, xlab = “”, ylab = “”)
axis(side = 4, line=0, at = pretty(range(density))) # Add second axis
mtext(“density”, side = 4, line = 3)
Ah, I see. In case of boxplots you would have to use the boxwex argument (e.g. boxwex = 0.5) instead of cex to change the width of the boxes.
Another (likely even better) alternative might be the at argument as explained in Example 8 of this tutorial: https://statisticsglobe.com/boxplot-in-r
Regards
Joachim
Hello, I really like your posts!
Is this a way to do this with ggplot?
L.
Hey Luis,
Thank you for the kind words!
As far as I know, the ggplot2 package restricts the usage of two y-axes. One way to draw two y-axes in a ggplot2 plot is shown here: https://statisticsglobe.com/draw-ggplot2-plot-with-two-y-axes-in-r The resulting plot is not the same as the plot shown in this tutorial though.
Regards,
Joachim
Hi Joachim,
I am wondering whether is it possible to fit a linear regression with predicted output (yi) and real output (yhati) into this code?
Hey Edwin,
Have you tried to use the abline function as explained here? https://statisticsglobe.com/abline-function-in-r/#example-6-draw-regression-line-to-plot-using-abline-function
Regards,
Joachim
Hi I was wondering if it is possible to put factors on the x-axis? So instead of -1.5, -1.0 etc, what function would you use to put for example “Day 1, Day 2” etc.
Hey Ben,
Please have a look here: https://statisticsglobe.com/replace-x-axis-values-in-r
Regards,
Joachim