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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:

**a, b:Â**single values that specify the intercept and slope of the line**h:Â**the y-value for the horizontal line**v:Â**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 *andÂ *yÂ *in aÂ dataset:

#define dataset data #plotxandyvalues in dataset plot(data$x, data$y, pch = 16)

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

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

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

#create scatterplot forxandyplot(data$x, data$y, pch = 16) #create horizontal line at mean value ofyabline(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)

**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 #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)

**How to Add Regression LinesÂ **

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 #create scatterplot ofxandyvalues plot(data$x, data$y, pch = 16) #fit a linear regression model to the data reg_model #add the fitted regression line to the scatterplot abline(reg_model, col="steelblue")

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 #create scatterplot ofxandyvalues plot(data$x, data$y, pch = 16) #fit a linear regression model to the data reg_model #define intercept and slope values a #intercept b #slope #add the fitted regression line to the scatterplot abline(a=a, b=b, col="steelblue")

Notice that this produces the same line as before.

You can find more R tutorials on this page.