# How To Annotate Clusters with Circle/Ellipse by a Variable in R ggplot2

• Last Updated : 24 Mar, 2022

In this article, we will discuss how to annotate Clusters with Circle/Ellipse by a categorical variable in the R Programming Language using the ggplot2 package.

To add a circle or ellipse around a cluster of data points, we use the geom_mark_circle() and geom_mark_ellipse() function of the ggforce package. This function automatically computes the circle/ellipse radius to draw around the cluster of points by categorical data.

First, we will plot the data in a scatter plot using the geom_point function of the ggplot2 package. We will use the color parameter of the aes() function to color the plot by a categorical variable group.

Syntax:

ggplot(df, aes( x, y ) ) + geom_point( aes( color ))

Arguments:

• df: determines the data frame to be used.
• x and y: determine the x-axis and y-axis variables respectively.
• color: determines the categorical variable for coloring the data point clusters.

Example:

Here, is a basic scatter plot made using the geom_point() function of the ggplot2 package. We have colored the plot by the categorical variable group.

## R

 # load library tidyverse library(tidyverse)   # set theme theme_set(theme_bw(16))   # create x and y vector xAxis <- rnorm(1000)                 yAxis <- rnorm(1000) + xAxis + 10      # create groups in variable using conditional # statements group <- rep(1, 1000)              group[xAxis > -1.5] <- 2 group[xAxis > -0.5] <- 3 group[xAxis > 0.5] <- 4 group[xAxis > 1.5] <- 5   # create sample data frame sample_data <- data.frame(xAxis, yAxis, group)   # create a scatter plot with points colored by # group ggplot(sample_data, aes(x = xAxis,              y = yAxis))+   geom_point(aes(color = as.factor(group)))

Output:

## Annotate circles around cluster:

To annotate a circle around a cluster of points by the group we use the geom_mark_circle() function of the ggforce package. To use this function we first install & import the ggforce package by using:

install. packages('ggforce')
library(ggforce)

Now, we will annotate the circle around a cluster of data points by using the geom_mark_circle() function.

Syntax:

ggplot(df, aes( x, y ) ) + geom_point( aes( color )) + geom_mark_circle( aes(color) )

Example:

Here, is a basic scatter plot with circles around a cluster of data points colored by a categorical variable group.

## R

 # load library tidyverse library(tidyverse) library(ggforce)   # set theme theme_set(theme_bw(16))   # create x and y vector xAxis <- rnorm(500)                 yAxis <- rnorm(1000) + xAxis + 10      # create groups in variable using conditional # statements group <- rep(1, 500)              group[xAxis > -1.5] <- 2 group[xAxis > -0.5] <- 3 group[xAxis > 0.5] <- 4 group[xAxis > 1.5] <- 5   # create sample data frame sample_data <- data.frame(xAxis, yAxis, group)   # create a scatter plot with points colored by group # circles are annotated using geom_mark_circle() function ggplot(sample_data, aes(x = xAxis,              y = yAxis))+   geom_point(aes(color = as.factor(group)))+   geom_mark_circle(aes(color = as.factor(group)), expand = unit(0.5,"mm"))+   theme(legend.position = "none")

Output:

## Annotate ellipses around cluster:

To annotate an ellipse around a cluster of points by the group we use the geom_mark_ellipse() function of the ggforce package. This function automatically computes the dimensions of the ellipse and overlays it on top of the scatter plot.

Syntax:

ggplot(df, aes( x, y ) ) + geom_point( aes( color )) + geom_mark_ellipse( aes(color) )

Example:

Here, is a basic scatter plot with ellipses around a cluster of data points colored by a categorical variable group.

## R

 # load library tidyverse library(tidyverse) library(ggforce)   # set theme theme_set(theme_bw(16))   # create x and y vector xAxis <- rnorm(500)                 yAxis <- rnorm(1000) + xAxis + 10      # create groups in variable using conditional # statements group <- rep(1, 500)              group[xAxis > -1.5] <- 2 group[xAxis > -0.5] <- 3 group[xAxis > 0.5] <- 4 group[xAxis > 1.5] <- 5   # create sample data frame sample_data <- data.frame(xAxis, yAxis, group)   # create a scatter plot with points colored by group # ellipses are annotated using geom_mark_ellipse() function ggplot(sample_data, aes(x = xAxis,              y = yAxis))+   geom_point(aes(color = as.factor(group)))+   geom_mark_ellipse(aes(color = as.factor(group)), expand = unit(0.5,"mm"))+   theme(legend.position = "none")

Output:

## Customizing the aesthetics

We can customize the aesthetics of the geom_mark_* function by using the color, fill, and alpha property of the aes() function.

Syntax:

ggplot(df, aes( x, y ) ) + geom_point( aes( color )) + geom_mark_ellipse( aes(color, fill, alpha) )

where,

• color: determines the color of the boundary of the circles or ellipses.
• fill: determines the background color of the circles or ellipses.
• alpha: determines the transparency of the circles or ellipses.

Example:

In this example, we will plot a  scatter plot overlayed by ellipses with a background colored by the group categorical variable.

## R

 # load library tidyverse library(tidyverse) library(ggforce)   # set theme theme_set(theme_bw(16))   # create x and y vector xAxis <- rnorm(500)                 yAxis <- rnorm(1000) + xAxis + 10      # create groups in variable using conditional # statements group <- rep(1, 500)              group[xAxis > -1.5] <- 2 group[xAxis > -0.5] <- 3 group[xAxis > 0.5] <- 4 group[xAxis > 1.5] <- 5   # create sample data frame sample_data <- data.frame(xAxis, yAxis, group)   # create a scatter plot with points colored by group # ellipses are annotated using geom_mark_ellipse() function ggplot(sample_data, aes(x = xAxis,              y = yAxis))+   geom_point(aes(color = as.factor(group)))+   geom_mark_ellipse(aes(fill = as.factor(group)), expand = unit(0.5,"mm"))+   theme(legend.position = "none")

Output:

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