Why do geom_density and stat_density(geom = "line") give different results?

Question

In the following illustration, why do geom_density and stat_density(geom = "line") give different results?

library(ggplot2)

df <- data.frame(
  x.values = c(
    rnorm(100, mean = 1, sd = 1),
    rnorm(100, mean = 4, sd = 1),
    rnorm(100, mean = 7, sd = 1),
    rnorm(100, mean = 10, sd = 1)
  ),
  mean.values = sort(rep(c(1, 4, 7, 10), 100))
)

p <- ggplot(df, aes(x = x.values, color = mean.values, group = mean.values))

p + geom_density()

p + stat_density(geom = "line")

Answer 1

It's a difference in the position argument. The default in stat_density is position = "stack", whilst with geom_density() it is position = "identity".

If you call p + stat_density(geom = "line", position = "identity") you get the same as geom_density():