R Data Visualisation Example - A Guide to Density plot by group in R using ggplot2

R Data Visualisation Example: Density plot by group using ggplot2

Import library

library(ggplot2)

Generate Sample Data for Plotting purposes

# Sample data
set.seed(007)

# Data
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)

head(df)

##           x group
## 1  1.430871     A
## 2 -3.795158     A
## 3 -3.041439     A
## 4 -2.618439     A
## 5 -3.456010     A
## 6 -3.420920     A

Basic BOX plot by group using ggplot2

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.

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

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.

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) 

Fill the density areas

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.

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

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.

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) 

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.

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.

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

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

Remove the legend

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

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