How to Add an Average Line to Plot in ggplot2

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))

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 here.