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

 

plot with two y axes in r

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:

 

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

 

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.

 

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18 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?

    Reply
    • 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:

      legend("topleft",
             legend = c("A", "B"),
             col = 2:3,
             pch = 16:17)

      Regards

      Joachim

      Reply
  • Maria Alejandra Munoz torres
    September 16, 2021 8:01 am

    Hi, nice code, How I can add space to not superpose the dot. Thanks

    Reply
    • Hey Maria,

      You may decrease the size of the points and triangles using the cex argument. For instance:

      plot(x, y1, pch = 16, col = 2, cex = 0.5)

      Regards

      Joachim

      Reply
  • Maria Alejandra Munoz torres
    September 16, 2021 5:37 pm

    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)

    Reply
  • Hello, I really like your posts!
    Is this a way to do this with ggplot?
    L.

    Reply
  • 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?

    Reply
  • 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.

    Reply
  • Hello! I am wondering if it is possible to change the ticks on the second y-axis? For example, I would like a range of 1690 to 1700 with ticks spaced apart 1 digit. (1690, 1691, 1692, etc)

    Reply
    • Hi Meg,

      You can do that by specifying the at argument within the axis function accordingly. In this example, I have used the pretty function to specify the axis values. However, you can assign basically any value you want to the at argument.

      You may also have a look here for more details on the axis function.

      Regards,
      Joachim

      Reply
  • Hey
    I’m having some problems with adding the right axis. I can add the text “BALF” but the values at the axis dosen’t show.
    This is my code:

    par(mar=c(4,4,4,4))
    boxplot(BALFogELF$ELF~BALFogELF$Study,pch=16,col=2)
    par(new=TRUE)
    boxplot(BALFogELF$ï..BALF~BALFogELF$Study,pch=17,col=3,axes=TRUE,xlab=””,ylab=””)
    axis(side=4,at=pretty(range(BALFogELF$ï..BALF)))
    mtext(“BALF”,side=4,line=3)

    I hope you can help 🙂

    Reply
    • Hey Trine,

      Could you illustrate the structure of the column BALFogELF$ï..BALF? What is returned when you print this column to the console?

      Regards,
      Joachim

      Reply
  • Thanks for the cool code! Is there any chance to change number facing to the graph? I mean a 180o rotation both numbers and axys name

    Reply
    • Hi Fausto,

      Thank you for the kind words, glad you like the code!

      I just returned from holidays, so unfortunately, I couldn’t respond to your question earlier. Are you still looking for help?

      Regards,
      Joachim

      Reply

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