How to label max value points in a faceted plot in R?

Question

I read someone had a similar question (ggplot2 and facet_grid : add highest value for each plot) but I still couldn't solve my problem.

Take this for example:

data.frame(x=rnorm(100),y=rnorm(100),z=rep(c("A","B"))) %>% ggplot(aes(x,y)) + geom_point() + facet_wrap(~z)

I want to label only the max y-value point in each plot. I wanted to use + geom_label_repel(aes(label=y)) but I ended up labelling all points.

I also tried with + geom_label(data=.[.$y==max(.$y),], aes(label=y)) where I thought . should be a placeholder for the argument left of the pipe operator but doing it this way does not work.

Quirky note: I also want to do this without assigning the data frame to a variable in the global environment so I'm using the pipe operator. Can we do this without assigning any variables?

Answer 1

You can do this by filtering your original data set and passing that as the "data" argument to your text geom. It looks a little strange (and you have to use the "." operator to reference the dplyr chain's data set, which I personally don't like), but it definitely works, and you don't have to externally reference the data.

set.seed(1222)

data.frame(x=rnorm(100),y=rnorm(100),z=rep(c("A","B"))) %>% 
  ggplot(aes(x,y)) + geom_point() + 
  geom_label(data = . %>% group_by(z) %>% filter(y == max(y)), aes(label = sprintf('%0.2f', y)), hjust = -0.5) +
  facet_wrap(~z)