如何从r中的ggplot中提取密度值

Question

如何提取每个密度图的值矩阵？

例如，我感兴趣的是，当重量= 71时，橙子、苹果、梨、香蕉的密度是多少？

下面是最小的例子：

library(ggplot2)
set.seed(1234)
df = data.frame(
  fruits = factor(rep(c("Orange", "Apple", "Pears", "Banana"), each = 200)),
  weight = round(c(rnorm(200, mean = 55, sd=5),
                 rnorm(200, mean=65, sd=5),
                 rnorm(200, mean=70, sd=5),
                 rnorm(200, mean=75, sd=5)))
)
dim(df) [1] 800   2

ggplot(df, aes(x = weight)) +
  geom_density() + 
  facet_grid(fruits ~ ., scales = "free", space = "free")

Answer 1

将绘图保存在变量中，构建数据结构并按ggplot_build面板分割数据。然后用 进行插值approx以获得新值。

g <- ggplot(df, aes(x = weight)) +
  geom_density() + 
  facet_grid(fruits ~ ., scales = "free", space = "free")

p <- ggplot_build(g)

# These are the columns of interest    
p$data[[1]]$x
p$data[[1]]$density
p$data[[1]]$PANEL

按面板拆分列表成员p$data[[1]]，但仅保留x和density值。然后循环遍历分割数据以按水果组进行插值。

sp <- split(p$data[[1]][c("x", "density")], p$data[[1]]$PANEL)

new_weight <- 71
sapply(sp, \(DF){
  with(DF, approx(x, density, xout = new_weight))
})
#  1          2          3           4         
#x 71         71         71          71        
#y 0.04066888 0.05716947 0.001319164 0.07467761

或者，在不预先拆分数据的情况下，使用by.

b <- by(p$data[[1]][c("x", "density")], p$data[[1]]$PANEL, \(DF){
  with(DF, approx(x, density, xout = new_weight))
})
do.call(rbind, lapply(b, as.data.frame))
#   x           y
#1 71 0.040668880
#2 71 0.057169474
#3 71 0.001319164
#4 71 0.074677607