How to use stat_summary to add a vertical line to a density plot in ggplot2

Question

I have a multi-panelled density plot with two density curves/panel. I'd like to add the mean value for each density curve as a vertical line in each panel (i.e., 2 lines/panel). How do I do this with stat_summary?

set.seed(42)
data_frame(site = rep(LETTERS[1:4], each = 25),
                 sex = rep(c("f", "m"), 50),
                 mass = rnorm(100, 50, 5)) %>% 
  ggplot(aes(x = mass, col = sex)) +
  geom_density() +
  stat_summary(fun = mean, geom = "vline") +
  facet_wrap(~site)
Error in `stat_summary()`:
! Problem while computing stat.
ℹ Error occurred in the 2nd layer.
Caused by error in `compute_layer()`:
! `stat_summary()` requires the following missing aesthetics: y
Run `rlang::last_trace()` to see where the error occurred.

When I add a term for y, I get another error message:

data_frame(site = rep(LETTERS[1:4], each = 25),
                 sex = rep(c("f", "m"), 50),
                 mass = rnorm(100, 50, 5)) %>% 
  ggplot(aes(x = mass, col = sex)) +
  geom_density() +
  stat_summary(fun = mean, geom = "vline", aes(y = 0)) +
  facet_wrap(~site)
Error in `stat_summary()`:
data_frame(site = rep(LETTERS[1:4], each = 25),
                 sex = rep(c("f", "m"), 50),
                 mass = rnorm(100, 50, 5)) %>% 
  ggplot(aes(x = mass, col = sex)) +
  geom_density() +
  stat_summary(fun = mean, geom = "vline", aes(y = 0)) +
  facet_wrap(~site)
! Problem while setting up geom.
ℹ Error occurred in the 2nd layer.
Caused by error in `compute_geom_1()`:
! `geom_vline()` requires the following missing aesthetics: xintercept
Run `rlang::last_trace()` to see where the error occurred.

and when I add a term for xintercept, then I get the value for every individual instead of the mean:

data_frame(site = rep(LETTERS[1:4], each = 25),
                 sex = rep(c("f", "m"), 50),
                 mass = rnorm(100, 50, 5)) %>% 
  ggplot(aes(x = mass, col = sex)) +
  geom_density() +
  stat_summary(fun = mean, geom = "vline", aes(xintercept = mass, y = 0)) +
  facet_wrap(~site)

stefan · Accepted Answer · 2024-03-22 03:27:08Z

4

The issue is that by default stat_summary will compute the mean of the variable mapped on y for each value of the variable mapped on x. Instead, to achieve your desired result you have to set orientation="y" to compute the mean of x, map a constant on y e.g. y = 0 and use after_stat() to map the computed mean on xintercept:

library(ggplot2)

set.seed(42)

dat <- data.frame(
  site = rep(LETTERS[1:4], each = 25),
  sex = rep(c("f", "m"), 50),
  mass = rnorm(100, 50, 5)
)

dat |>
  ggplot(aes(x = mass, col = sex)) +
  geom_density() +
  stat_summary(
    aes(xintercept = after_stat(x), y = 0),
    fun = mean, geom = "vline", orientation = "y"
  ) +
  facet_wrap(~site)

answered Mar 22, 2024 at 3:27

stefan

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