Use stat_summary to annotate plot with number of observations

Question

How can I use `stat_summary` to label a plot with `n = x` where is `x` a variable? Here's an example of the desired output: I can make that above plot with this rather inefficient code: ``` nlabels <- sapply(1:length(unique(mtcars$cyl)), function(i) as.vector(t(as.data.frame(table(mtcars$cyl))[,2][[i]]))) ggplot(mtcars, aes(factor(cyl), mpg, label=rownames(mtcars))) + geom_boxplot(fill = "grey80", colour = "#3366FF") + geom_text(aes(x = 1, y = median(mtcars$mpg[mtcars$cyl==sort(unique(mtcars$cyl))[1]]), label = paste0("n = ",nlabels[[1]]) )) + geom_text(aes(x = 2, y = median(mtcars$mpg[mtcars$cyl==sort(unique(mtcars$cyl))[2]]), label = paste0("n = ",nlabels[[2]]) )) + geom_text(aes(x = 3, y = median(mtcars$mpg[mtcars$cyl==sort(unique(mtcars$cyl))[3]]), label = paste0("n = ",nlabels[[3]]) )) ``` This is a follow up to this question: How to add a number of observations per group and use group mean in ggplot2 boxplot? where I can use `stat_summary` to calculate and display the number of…

Accepted Answer

You can make your own function to use inside the `stat_summary()`. Here `n_fun` calculate place of y value as `median()` and then add `label=` that consist of `n=` and number of observations. It is important to use `data.frame()` instead of `c()` because `paste0()` will produce character but `y` value is numeric, but `c()` would make both character. Then in `stat_summary()` use this function and `geom="text"`. This will ensure that for each x value position and label is made only from this level's data.

```
n_fun <- function(x){
  return(data.frame(y = median(x), label = paste0("n = ",length(x))))
}

ggplot(mtcars, aes(factor(cyl), mpg, label=rownames(mtcars))) +
  geom_boxplot(fill = "grey80", colour = "#3366FF") + 
  stat_summary(fun.data = n_fun, geom = "text")
```

Use stat_summary to annotate plot with number of observations

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