How to create a marginal plot with ggplot2 in R

Recipe Objective

A marginal plot is an extension of scatter plot which not only shows the relationship between two variables but also shows the individual distributions such as Histogram, Box plots of the variables. ​

In this recipe we are going to use ggplot2 as well as ggExtra package to plot the required Marginal plot. ggplot2 package is based on the book Grammar of Graphics by Wilkinson. This package provides flexibility while incorporating different themes and plot specification with a high level of abstraction. The package mainly uses aesthetic mapping and geometric objects as arguments. Different types of geometric objects include: ​

1. geom_point() - for plotting points
2. geom_bar() - for plotting bar graph
3. geom_line() - for plotting line chart
4. geom_histogram() - for plotting histogram

The basic syntax of gggplot2 plots is: ​

ggplot(data, mapping = aes(x =, y=)) + geometric object ​

where: ​

1. data : Dataframe that is used to plot the chart
2. mapping = aes() : aesthetic mapping which deals with controlling axis (x and y indicates the different variables)
3. geometric object : Indicates the code for typeof plot you need to visualise.

ggExtra Package is mainly used to further enhance the built-in features of "ggplot2" by providing us with various functions to create Marginal plots especially Histogram and Boxplot.

This recipe demonstrates how to make a Marginal Plots using ggplot2 and ggExtra.

STEP 1: Loading required library and dataset

We will take an example of normal distribution along x and y axis and showcase the individual histogram and boxplot of the same.

x	y
20.08595	27.34024
24.57561	33.94111
31.76342	33.02787
31.81128	15.18382
23.61155	31.49135
...	...
24.98564	27.50006
25.33502	32.35901
26.72924	29.06489
23.44376	22.31388
20.21346	19.30083

STEP 2: Plotting a scatter plot using ggplot

We use geometric object as geom_point() to plot a scatter plot (x vs y)

Note:

1. The + sign in the syntax earlier makes the code more readable and enables R to read further code without breaking it.
2. We also use labs() function to give a title to the graph

STEP 3: Marginal Plot- Histogram and Boxplot

We use "ggExtra::ggMarginal()" function to plot a marginal plot.

syntax: ggExtra::ggMarginal(x , type = )

where:

1. x = plot on which the marginal plot is created
2. type = type of marginal plot