The abline() function in R can be used to add one or more straight lines to a plot in R.

This function uses the following syntax:

abline(a=NULL, b=NULL, h=NULL, v=NULL, …)

where:

  • a, b:single values that specify the intercept and slope of the line
  • h:the y-value for the horizontal line
  • v:the x-value for the vertical line

The following examples show how to use this function in practice.

How to Add Horizontal Lines

The basic code to add a horizontal line to a plot in R is: abline(h = some value)

Suppose we have the following scatterplot that displays the values for x andyin a dataset: 

                                      #define dataset                    data <- data.frame(x = c(1, 1, 2, 3, 4, 4, 5, 6, 7, 7, 8, 9, 10, 11, 11),                    y = c(13, 14, 17, 12, 23, 24, 25, 25, 24, 28, 32, 33, 35, 40, 41))                    #plot                      x                      and                      y                      values in dataset                    plot(data$x, data$y, pch = 16)

Scatterplot in R

To add a horizontal line at the value y = 20, we can use the following code: 

                  abline(h = 20, col = 'coral2', lwd = 2)

Example of abline() in R

The following code illustrates how to add a horizontal solid line at the mean value ofyalong with two horizontal dashed lines at one standard deviation above and below the mean value: 

                                      #create scatterplot for                      x                      and                      y                                        plot(data$x, data$y, pch = 16)                    #create horizontal line at mean value of                      y                    abline(h = mean(data$y), lwd = 2)                    #create horizontal lines at one standard deviation above and below the mean value                    abline(h = mean(data$y) + sd(data$y), col = 'steelblue', lwd = 3, lty = 2) abline(h = mean(data$y) - sd(data$y), col = 'steelblue', lwd = 3, lty = 2)

Horizontal lines in R using abline() function

How to Add Vertical Lines

The basic code to add a vertical line to a plot in R is: abline(v = some value)

The following code illustrates how to add a vertical line at the mean value on a histogram: 

                                      #make this example reproducible                    set.seed(0)                    #create dataset with 1000 random values normally distributed with mean = 10, sd = 2                    data <- rnorm(1000, mean = 10, sd = 2)                    #create histogram of data values                    hist(data, col = 'steelblue')                    #draw a vertical dashed line at the mean value                    abline(v = mean(data), lwd = 3, lty = 2)

Abline on a histogram in R

How to Add Regression Lines

The basic code to add a simple linear regression line to a plot in R is: abline(model)

The following code illustrates how to add a fitted linear regression line to a scatterplot: 

                                      #define dataset                    data <- data.frame(x = c(1, 1, 2, 3, 4, 4, 5, 6, 7, 7, 8, 9, 10, 11, 11),                    y = c(13, 14, 17, 12, 23, 24, 25, 25, 24, 28, 32, 33, 35, 40, 41))                    #create scatterplot of                      x                      and                      y                      values                    plot(data$x, data$y, pch = 16)                    #fit a linear regression model to the data                    reg_model <- lm(y ~ x, data = data)                    #add the fitted regression line to the scatterplot                    abline(reg_model, col="steelblue")

Abline() of a regression line in R

Note that we simply need a value for the intercept and the slope to fit a simple linear regression line to the data using the abline() function.

Thus, another way of using abline() to add a regression line is to explicitly specify the intercept and slope coefficients of the regression model: 

                                      #define dataset                    data <- data.frame(x = c(1, 1, 2, 3, 4, 4, 5, 6, 7, 7, 8, 9, 10, 11, 11),                    y = c(13, 14, 17, 12, 23, 24, 25, 25, 24, 28, 32, 33, 35, 40, 41))                    #create scatterplot of                      x                      and                      y                      values                    plot(data$x, data$y, pch = 16)                    #fit a linear regression model to the data                    reg_model <- lm(y ~ x, data = data)                    #define intercept and slope values                    a <- coefficients(reg_model)[1]                    #intercept                    b <- coefficients(reg_model)[2]                    #slope                    #add the fitted regression line to the scatterplot                    abline(a=a, b=b, col="steelblue")

Abline() of a regression line in R with specific intercept and slope

Notice that this produces the same line as before.

You can find more R tutorials on this page.