如何在ggplot2密度曲线下遮蔽特定区域? [英] How to shade specific region under ggplot2 density curve?

问题描述

(这与 jlhoward 的回答几乎相同，但从 ggplot 中获取计算值).

Related post here

My intention is to shade area an area underneath the density curve that lie between two points. In this example, I would like to shade the areas between the values .25 and .5.

I have been able to plot my density curve with the following:

    setwd("D:/Workspace")

    # -- create dataframe

    coursename <- c('Math','Math','Math','Math','Math')
    value <- c(.12, .4, .5, .8, .9)
    df <- data.frame(coursename, value)

    library(ggplot2)

    density_plot <- ggplot(aes(x=value, colour=coursename, fill=coursename), data=df) +
                      geom_density(alpha=.3) +
                      geom_vline(aes(xintercept=.5), colour="blue", data=df, linetype="dashed", size=1) +
                      scale_x_continuous(breaks=c(0, .25, .5, .75, 1), labels=c("0", ".25", ".5", ".75", "1")) +
                      coord_cartesian(xlim = c(0.01, 1.01)) +
                      theme(axis.title.y=element_blank(), axis.text.y=element_blank()) +
                      ggtitle("sample data")

    density_plot

I've tried using the following code to shade the area between .25 and .5:

    x1 <- min(which(df$value >=.25))
    x2 <- max(which(df$value <=.5))

    with(density_plot, polygon(x=c(x[c(x1,x1:x2,x2)]), y=c(0, y[x1:x2], 0), col="gray"))

But it just generates the following error:

Error in xy.coords(x, y) : object 'y' not found

解决方案

Or use ggplot2 against itself!

coursename <- c('Math','Math','Math','Math','Math')
value <- c(.12, .4, .5, .8, .9)
df <- data.frame(coursename, value)

library(ggplot2)

ggplot() +
  geom_density(data=df, 
               aes(x=value, colour=coursename, fill=coursename),
               alpha=.3) +
  geom_vline(data=df, 
             aes(xintercept=.5), 
             colour="blue", linetype="dashed", size=1) +
  scale_x_continuous(breaks=c(0, .25, .5, .75, 1), 
                     labels=c("0", ".25", ".5", ".75", "1")) +
  coord_cartesian(xlim = c(0.01, 1.01)) +
  theme(axis.title.y=element_blank(), 
        axis.text.y=element_blank()) +
  ggtitle("sample data") -> density_plot

density_plot

dpb <- ggplot_build(density_plot)

x1 <- min(which(dpb$data[[1]]$x >=.25))
x2 <- max(which(dpb$data[[1]]$x <=.5))

density_plot +
  geom_area(data=data.frame(x=dpb$data[[1]]$x[x1:x2],
                       y=dpb$data[[1]]$y[x1:x2]),
            aes(x=x, y=y), fill="grey")

(this pretty much does the same thing as jlhoward's answer but grabs the calculated values from ggplot).

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