You can use the following basic syntax to add a line that represents the average value in a plot in ggplot2:

ggplot(df, aes(x=x, y=y)) +
  geom_point() +
  geom_hline(yintercept = mean(df$y, na.rm=TRUE))

The following example shows how to use this syntax in practice.

Example: Add Average Line to Plot in ggplot2

Suppose we have the following data frame in R:

#create data frame
df <- data.frame(x=c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12),
                 y=c(2, 5, 6, 5, 7, 8, 10, 12, 10, 9, 11, 15))

#view head of data frame
head(df)

  x y
1 1 2
2 2 5
3 3 6
4 4 5
5 5 7
6 6 8

We can use the following code to create a scatter plot of x vs. y and add a horizontal line that represents the average y-value:

library(ggplot2)

#create scatter plot with average line to represent average y-value
ggplot(df, aes(x=x, y=y)) +
    geom_point() +
    geom_hline(yintercept = mean(df$y, na.rm=TRUE))

We can see that an average line has been added to the plot just above the y-value of 8.

 If we calculate the average y-value, we’ll find that it’s 8.333:

#calculate average y-value
mean(df$y, na.rm=TRUE)

[1] 8.333333

Note that we can also use the color, lty, and lwd arguments to specify the color, line type, and line width of the average line, respectively:

library(ggplot2)

#create scatter plot with custom average line
ggplot(df, aes(x=x, y=y)) +
    geom_point() +
    geom_hline(yintercept = mean(df$y, na.rm=TRUE), color='blue', lty='dashed', lwd=2)

The average line is now blue, dashed, and has a line width of 2.

You can find the complete online documentation for the geom_hline() function .

Additional Resources

The following tutorials explain how to perform other common tasks in ggplot2: