Density plot by group in ggplot2

Sample data

The following data frame contains three normal distributions with different mean and standard deviation separated by group. This data will be used in all the examples of this tutorial.

# Data
set.seed(5)
x <- c(rnorm(200, mean = -2, 1.5),
       rnorm(200, mean = 0, sd = 1),
       rnorm(200, mean = 2, 1.5))
group <- c(rep("A", 200), rep("B", 200), rep("C", 200))
df <- data.frame(x, group)

Density plot by group with `geom_density`

In order to create a density plot by group in ggplot you need to input the numerical variable and specify the grouping variable in color (or colour) argument inside aes and use geom_density function.

# install.packages("ggplot2")
library(ggplot2)

# Basic density plot in ggplot2
ggplot(df, aes(x = x, colour = group)) +
  geom_density()

Density plot by group in ggplot2

The default color palette for the lines can be customized with scale_color_manual (or scale_color_brewer, for instance). You can also change the width and line type of the curves with lwd and linetype, respectively.

# install.packages("ggplot2")
library(ggplot2)

cols <- c("#F76D5E", "#FFFFBF", "#72D8FF")

# Basic density plot in ggplot2
ggplot(df, aes(x = x, colour = group)) +
  geom_density(lwd = 1.2, linetype = 1) + 
  scale_color_manual(values = cols)

Change colors of density curves by group in ggplot

Fill the density areas

Fill area density chart by group ggplot2

If you also set the categorical variable to fill inside aes the areas under the curves will be filled with a color. Note that you can remove colour = group or set a custom color if you don’t want to color the lines by group.

# install.packages("ggplot2")
library(ggplot2)

# Basic density plot in ggplot2
ggplot(df, aes(x = x, colour = group, fill = group)) +
  geom_density()

ggplot density plot by group with transparency

Transparency and custom colors

You can modify the transparency of the areas with the alpha argument of geom_density and set custom colors with scale_fill_manual.

# install.packages("ggplot2")
library(ggplot2)

cols <- c("#F76D5E", "#FFFFBF", "#72D8FF")

# Basic density plot in ggplot2
ggplot(df, aes(x = x, fill = group)) +
  geom_density(alpha = 0.7) + 
  scale_fill_manual(values = cols)

ggplot2 density comparison chart with custom colors

Remove the lines

If you want to get rid of the lines and only show the area you can set color = NA inside geom_density.

# install.packages("ggplot2")
library(ggplot2)

cols <- c("#F76D5E", "#FFFFBF", "#72D8FF")

# Density areas without lines
ggplot(df, aes(x = x, fill = group)) +
  geom_density(alpha = 0.8, color = NA) + 
  scale_fill_manual(values = cols)

Legend customization

Custom title

The default title (the name of the categorical variable) can be customized with the following code.

# install.packages("ggplot2")
library(ggplot2)

# Basic density plot in ggplot2
ggplot(df, aes(x = x, fill = group)) +
  geom_density() + 
  guides(fill = guide_legend(title = "Title"))

Density plot by group in ggplot2 legend title

Custom labels

You can also set custom key labels if you don’t want to use the name of your categories. Note that you can use scale_fill_hue if you only want to change the labels, but use the labels argument of scale_fill_manual if you also need to change the fill colors.

# install.packages("ggplot2")
library(ggplot2)

# Custom legend labels
ggplot(df, aes(x = x, fill = group)) +
  geom_density() + 
  scale_fill_hue(labels = c("G1", "G2", "G3"))

Legend key labels of density plot in ggplot

Remove the legend

If you want to get rid of the legend, which appears by default, you can set its position to "none".

# install.packages("ggplot2")
library(ggplot2)

# Basic density plot in ggplot2
ggplot(df, aes(x = x, fill = group)) +
  geom_density() + 
  theme(legend.position = "none")

Density plot by group in ggplot2 without legend