Mirror density chart with ggplot2


A density plot is a representation of the distribution of a numeric variable. Comparing the distribution of 2 variables is a common challenge that can be tackled with the mirror density chart: 2 density charts are put face to face what allows to efficiently compare them. Here is how to build it with ggplot2 library.

Density Section Density theory

Density with geom_density

A density chart is built thanks to the geom_density geom of ggplot2 (see a basic example). It is possible to plot this density upside down by specifying y = -..density... It is advised to use geom_label to indicate variable names.

# Libraries
library(ggplot2)
library(hrbrthemes)

# Dummy data
data <- data.frame(
  var1 = rnorm(1000), 
  var2 = rnorm(1000, mean=2)
)
 
# Chart
p <- ggplot(data, aes(x=x) ) + 
  # Top
  geom_density( aes(x = var1, y = ..density..), fill="#69b3a2" ) + 
  geom_label( aes(x=4.5, y=0.25, label="variable1"), color="#69b3a2") +
  # Bottom
  geom_density( aes(x = var2, y = -..density..), fill= "#404080") +
  geom_label( aes(x=4.5, y=-0.25, label="variable2"), color="#404080") +
  theme_ipsum() +
  xlab("value of x")

#p

Histogram with geom_histogram

Of course it is possible to apply exactly the same technique using geom_histogram instead of geom_density to get a mirror histogram:

# Chart
p <- ggplot(data, aes(x=x) ) + 
  geom_histogram( aes(x = var1, y = ..density..), fill="#69b3a2" ) + 
  geom_label( aes(x=4.5, y=0.25, label="variable1"), color="#69b3a2") +
  geom_histogram( aes(x = var2, y = -..density..), fill= "#404080") +
  geom_label( aes(x=4.5, y=-0.25, label="variable2"), color="#404080") +
  theme_ipsum() +
  xlab("value of x")

#p

Related chart types
Violin
Density
Histogram
Boxplot
Ridgeline



Contact

This document is a work by Yan Holtz. Any feedback is highly encouraged. You can fill an issue on Github, drop me a message on Twitter, or send an email pasting yan.holtz.data with gmail.com.

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