如何用ggplot在一个区间内填充密度图？

Question

The code below fills each of the two densities with color under the area of the curve:

library(ggplot2)

#fake data
dat <- data.frame(dens = c(rnorm(100), rnorm(100, 2, 0.5))
                   , group = rep(c("C", "P"), each = 100))

#fill the area under the curve
ggplot(dat, aes(x = dens, fill = group)) + geom_density(alpha = 0.75)

How can I achieve the following two goals?

1) Only fill each curve within a specified interval. For example, interval [-1.5, 2.0] for group 'C' and [0.5, 2.8] for group 'P'.

2) Add a vertical segment (from x-axis to the curve) for each density. For example, at x=0.2 for group 'C' and at x=1.9 for group 'P'.

Answer 1

To get you stared, here's your first question:

library(dplyr)
library(purrr)
library(tidyr)
library(ggplot2)

as.data.frame.density <- function(x) data.frame(x = x$x, y = x$y)

densities <- dat %>% 
  group_nest(group) %>% 
  mutate(dens = map(data, ~as.data.frame(density(.$dens)))) %>% 
  unnest(dens)

ggplot(densities, aes(x = x, y = y, group = group)) + 
  geom_density(stat = 'identity') +
  geom_density(
    aes(fill = group),
    . %>% filter((group == "C" & between(x, -1.5, 2.0)) | (group == "P" & between(x, 0.5, 2.8))),
    stat = 'identity',
    alpha = 0.75
  )

There are other ways of calculating the densities per group, using dplyr is just one way. It is probably good to set an equal bandwidth to the two density estimations.

Adding the segments is similar to this approach, you just need to find the correct values in the densities data.frame.