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

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, â€¦)
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where:
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• 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 #plot x and y values 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 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)```

### 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 of x and y values
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 of x and y values
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.