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# How to Add a Horizontal Line to a Plot Using ggplot2

You can quickly add horizontal lines to ggplot2 plots using the geom_hline() function, which uses the following syntax:

geom_hline(yintercept, linetype, color, size)

where:

• yintercept: Location to add line on the y-intercept.
• linetype: Line style. Default is â€˜solidâ€™ but you can specify â€˜twodashâ€™, â€˜longdashâ€™, â€˜dottedâ€™, â€˜dotdashâ€™, â€˜dashedâ€™, or â€˜blank.â€™
• color: Color of the line.
• size: Width of the line.

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

### Add a Single Horizontal Line to a Plot

The following code shows how to add a single horizontal line to a plot:

```library(ggplot2)

#create data frame
df frame(x=c(1, 3, 3, 4, 5, 5, 6, 9, 12, 15),
y=c(13, 14, 14, 12, 17, 21, 22, 28, 30, 31))

#create scatterplot with horizontal line at y=20
ggplot(df, aes(x=x, y=y)) +
geom_point() +
geom_hline(yintercept=20)
```

### Add Multiple Horizontal Lines to Plots

The following code shows how to add multiple horizontal lines to a plot:

```library(ggplot2)

#create data frame
df frame(x=c(1, 3, 3, 4, 5, 5, 6, 9, 12, 15),
y=c(13, 14, 14, 12, 17, 21, 22, 28, 30, 31))

#create scatterplot with horizontal lines at y = 10, 20, 30
ggplot(df, aes(x=x, y=y)) +
geom_point() +
geom_hline(yintercept=c(10, 20, 30))```

### Customize Horizontal Lines

The following code shows how to customize horizontal lines on a plot:

```library(ggplot2)

#create data frame
df frame(x=c(1, 3, 3, 4, 5, 5, 6, 9, 12, 15),
y=c(13, 14, 14, 12, 17, 21, 22, 28, 30, 31))

#create scatterplot with customized horizontal lines
ggplot(df, aes(x=x, y=y)) +
geom_point() +
geom_hline(yintercept=c(20, 30), linetype='dashed', color=c('blue', 'red'))```