You want to add lines to a plot.

For horizontal and vertical lines, use geom_hline() and geom_vline(), and for angled lines, use geom_abline() (Figure 7-7). For this example, we’ll use the heightweight data set:

library(gcookbook) # For the data set

p <- ggplot(heightweight, aes(x=ageYear, y=heightIn, colour=sex)) + geom_point()

# Add horizontal and vertical lines
p + geom_hline(yintercept=60) + geom_vline(xintercept=14)

# Add angled line
p + geom_abline(intercept=37.4, slope=1.75)

Figure 7-7. Left: horizontal and vertical lines; right: angled line

The previous examples demonstrate setting the positions of the lines manually, resulting in one line drawn for each geom added. It is also possible to map values from the data to xintercept, yintercept, and so on, and even draw them from another data frame.