Definition

A radar or spider or web chart is a two-dimensional chart type designed to plot one or more series of values over multiple quantitative variables. Each variable has its own axis, all axes are joined in the center of the figure.

Let’s consider the exam results of a student. He has a mark ranging from 0 to 20 for ten topics like math, sports, statistics, and so on. The radar chart provides one axis for each topic. The shape allows you to see which topics the student performed well or poorly in.

# Libraries
library(tidyverse)
library(viridis)
library(patchwork)
library(hrbrthemes)
library(fmsb)
library(colormap)

# Create data
set.seed(1)
data <- as.data.frame(matrix( sample( 2:20 , 10 , replace=T) , ncol=10))
colnames(data) <- c("math" , "english" , "biology" , "music" , "R-coding", "data-viz" , "french" , "physic", "statistic", "sport" )

# To use the fmsb package, I have to add 2 lines to the dataframe: the max and min of each topic to show on the plot!
data <- rbind(rep(20,10) , rep(0,10) , data)

# Custom the radarChart !
par(mar=c(0,0,0,0))
radarchart( data, axistype=1,

  #custom polygon
  pcol=rgb(0.2,0.5,0.5,0.9) , pfcol=rgb(0.2,0.5,0.5,0.5) , plwd=4 ,

  #custom the grid
  cglcol="grey", cglty=1, axislabcol="grey", caxislabels=seq(0,20,5), cglwd=0.8,

  #custom labels
  vlcex=0.8
  )

Variation

In the previous chart, only one series is plotted, showing how one student performed. A common task is to compare several individuals. With only a few series it is possible to show every group on the same chart. Here it is quite obvious that Shirley globally performed better than Sonia, except in sport, english and R-coding.

# Create data: note in High school for Jonathan:
set.seed(1)
data <-as.data.frame(matrix( c( sample( 2:20 , 10 , replace=T), sample( 2:9 , 10 , replace=T)) , ncol=10, byrow=TRUE))
colnames(data) <- c("math" , "english" , "biology" , "music" , "R-coding", "data-viz" , "french" , "physic", "statistic", "sport" )
data[2,2]=19

# To use the fmsb package, I have to add 2 lines to the dataframe: the max and min of each topic to show on the plot!
data <-rbind(rep(20,10) , rep(0,10) , data)

# Prepare color
colors_border=c( rgb(0.2,0.5,0.5,0.9), rgb(0.8,0.2,0.5,0.9)  )
colors_in=c( rgb(0.2,0.5,0.5,0.4), rgb(0.8,0.2,0.5,0.4)  )

# Custom the radarChart !
radarchart( data, axistype=1,

  #custom polygon
  pcol=colors_border , pfcol=colors_in , plwd=4, plty=1 ,

  #custom the grid
  cglcol="grey", cglty=1, axislabcol="grey", caxislabels=seq(0,20,5), cglwd=1.1,

  #custom labels
  vlcex=0.8
  )

# Legend
legend(x=0.85, y=1, legend = c("Shirley", "Sonia"), bty = "n", pch=20 , col=colors_border , text.col = "black", cex=0.9, pt.cex=1.6)

With more than two or three series, it is a good practice to use small multiples to avoid a cluttered figure. Each student is represented on their own radar chart. It is easy to understand the features of a specific individual, and looking for similarity in shapes allows you to find students with similar features.

# Create data: note in High school for Jonathan:
set.seed(1)
data <-as.data.frame(matrix( sample( 2:20 , 60 , replace=T) , ncol=10, byrow=TRUE))
colnames(data) <- c("math" , "english" , "biology" , "music" , "R-coding", "data-viz" , "french" , "physic", "statistic", "sport" )

# To use the fmsb package, I have to add 2 lines to the dataframe: the max and min of each topic to show on the plot!
data <-rbind(rep(20,10) , rep(0,10) , data)

# Prepare color
colors_border=colormap(colormap=colormaps$viridis, nshades=6, alpha=1)
colors_in=colormap(colormap=colormaps$viridis, nshades=6, alpha=0.3)

# Prepare title
mytitle <- c("Max", "George", "Xue", "Tom", "Alice", "bob")

# Split the screen in 6 parts
par(mar=rep(0.8,4))
par(mfrow=c(2,3))

# Loop for each plot
for(i in 1:6){

  # Custom the radarChart !
  radarchart( data[c(1,2,i+2),], axistype=1,

    #custom polygon
    pcol=colors_border[i] , pfcol=colors_in[i] , plwd=4, plty=1 ,

    #custom the grid
    cglcol="grey", cglty=1, axislabcol="grey", caxislabels=seq(0,20,5), cglwd=0.8,

    #custom labels
    vlcex=0.8,

    #title
    title=mytitle[i]
    )
}