How to Use the abline() Function in R to Add Straight Lines to Plots

How to use the abline() function in R

This tutorial explains how to use the abline() function in R to add one or more straight lines to a plot in R.

The abline() Function in R

The abline() function in R can be used to add one or more straight lines to a plot in R. The basic syntax is of abline() is as follows:

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

For full documentation of the abline() function, check out the R Documentation page.

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 and in 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:

Note that lwd = 2 specifies that we want the line width to be equal to 2 (default = 1).

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 of along with two horizontal dashed lines at one standard deviation above and below the mean value:

Note that lty = 2 specifies that we want the line to be dashed.

#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(reg_model)

where reg_model is a fitted regression line created by using the lm() function.

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 (although a more tedious 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.

Further Reading:
A Quick and Easy Way to Plot a Linear Regression Line in R

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