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Overlaying density plots in ggplot2 is a great way to compare the distributions of different variables in a single graph. It can be done by using the geom_density() function and specifying the fill argument to differentiate between the two variables. An example of this is shown in the code below. Here, the density plots are overlaid using different colors and grouped by the gender variable. The resulting graph provides a visual representation of the differences in the distributions of the two variables.
A density plot is a useful way to visualize the distribution of values in a dataset.
Often you may want to visualize the density plots of several variables at once. Fortunately, this is easy to do using the data visualization package in R with the following syntax:
ggplot(data, aes(x=value, fill=variable)) + geom_density(alpha=.25)
The alpha argument controls the opacity of each density plot. It’s important to set this value below 1 so that you can see each density plot when they overlay each other.
The following step-by-step example shows how to use this syntax in practice.
Step 1: Create the Data
First, let’s create a fake dataset with three variables:
#make this example reproducible set.seed(1) #create data df <- data.frame(var1=rnorm(1000, mean=0, sd=1), var2=rnorm(1000, mean=0, sd=3), var3=rnorm(1000, mean=3, sd=2)) #view first six rows of data head(df) var1 var2 var3 1 -0.6264538 3.4048953 1.2277008 2 0.1836433 3.3357955 -0.8445098 3 -0.8356286 -2.6123329 6.2394015 4 1.5952808 0.6321948 4.0385398 5 0.3295078 0.2081869 2.8883001 6 -0.8204684 -4.9879466 4.3928352
Step 2: Convert the Data from Wide to Long
Next, we need to convert the data from a wide format to a long format to make it compatible with ggplot2:
library(reshape) #convert from wide format to long format data <- melt(df) #view first six rows head(data) variable value 1 var1 -0.6264538 2 var1 0.1836433 3 var1 -0.8356286 4 var1 1.5952808 5 var1 0.3295078 6 var1 -0.8204684
Step 3: Create the Overlaying Density Plots
Lastly, we can create the overlaying density plots:
library(ggplot2) #create overlaying density plots ggplot(data, aes(x=value, fill=variable)) + geom_density(alpha=.25)
Feel free to adjust the alpha value to make the density plots more or less transparent.
For example, here’s what the plots would look like if we increased the alpha value:
library(ggplot2) #create overlaying density plots ggplot(data, aes(x=value, fill=variable)) + geom_density(alpha=.7)
