ggplot filling each standard deviation a different color

Question

I've been looking around online and I can't find a clear answer how to fill different sections under the normal distribution using ggplot and stat_function --> dnorm.

#Parameters 

mu = 18.7
sigma = 36.4
n = 168
x = c(mu - 4*sigma/sqrt(n), mu + 4*sigma/sqrt(n))

Here's what I have for the plot

ggplot(data, aes(x)) + 
  stat_function(fun = dnorm,
                args = list(mean = mu, sd = sigma/sqrt(n)), 
                geom = "area", 
                fill = "steelblue")

What I want is to fill the first standard deviation one color, the second another color, and so on. I've also noticed that the curve does not appear smooth. Any reason why?

Thanks in advance.

user_903061 · Answer

To get different colors with stat_function, you can use the xlim argument, e.g.,

ggplot(data.frame(x), aes(x)) + 
  stat_function(fun = dnorm,
                args = list(mean = mu, sd = sigma/sqrt(n)), 
                geom = "area", 
                fill = "steelblue", n = 1001) +
    stat_function(fun = dnorm,
                args = list(mean = mu, sd = sigma/sqrt(n)), 
                geom = "area", 
                fill = "chartreuse", n = 1001, xlim = c(mu - sigma / sqrt(n), mu))

That could get tedious, so if you want every standard deviation different it would be simpler to generate the function data and use geom_area directly:

sim_dat = data.frame(x = seq(x[1], x[2], length.out = 1001))
sim_dat$y = dnorm(sim_dat$x, mean = mu, sd = sigma / sqrt(n))
sim_dat$sds = cut(sim_dat$x, breaks = c(-Inf, mu + sigma / sqrt(n) * (-3:3), Inf), labels = setdiff(-4:4, 0))

ggplot(sim_dat, aes(x, y, fill = sds)) + geom_area()

ggplot filling each standard deviation a different color

1 Answers