How to access a mapped variable from aes() in geom_hline()?

December 14, 2023

I want to use the variable mapped to y in geom_hline(). Basically, I want to achieve the same behavior as in:

ggplot(mtcars, aes(x = hp, y = qsec)) +
  geom_point() +
  geom_hline(yintercept = mean(mtcars$qsec))

But I don’t want to "repeat" mtcars$qsec for the yintercept parameter.

I tried using something like yintercept = mean({{ y }}) but this gives me the "Error: object ‘y’ not found". To be honest, I do not fully understand when we can use these curly brackets.

Is there a way to do this? Thanks for your help!

>Solution :

geom_hline does not have access to the y aesthetic. You could do:

ggplot(mtcars, aes(x = hp, y = qsec)) +
  geom_point() +
  geom_hline(aes(yintercept = mean(qsec)))

From the comments, it appears that the issue is having to repeat the code for several geom_hlines. One can actually pass several values to geom_hline in a single call, so the easiest way to do this might be to summarize the data being passed to a single geom_hline call:

ggplot(mtcars, aes(x = hp, y = qsec)) +
  geom_point() +
  geom_hline(data = . %>% 
               reframe(mean = mean(qsec), 
                       sd = sd(qsec),
                       yintercept = seq(mean - 2*sd, mean + 2*sd, sd),
                       color = factor(c('-2sd', '-1sd', 'mean', '+1sd', '+2sd'),
                                      c('-2sd', '-1sd', 'mean', '+1sd', '+2sd'))
               ), aes(yintercept = yintercept, color = color)) +
  scale_color_manual(NULL, values = c('#FF000040', '#FF000080', 'red', 
                                      '#FF000080', '#FF000040'))