radar chart

R code of the ggplot2 example

library(ggplot2)
library(dplyr)
library(tidyr)
library(palmerpenguins)

## Each measure rescaled from 0 to 1 over all the birds, then averaged by
## species: the spokes compare species without stretching their differences.
measures <- c(bill_length_mm = "Bill length", bill_depth_mm = "Bill depth",
              flipper_length_mm = "Flipper length", body_mass_g = "Body mass")
means <- penguins |>
  pivot_longer(all_of(names(measures)), names_to = "measure", values_to = "value") |>
  filter(!is.na(value)) |>
  group_by(measure) |>
  mutate(value = (value - min(value)) / (max(value) - min(value))) |>
  group_by(species, measure) |>
  summarise(value = mean(value), .groups = "drop")

## Polygons drawn in plain x-y coordinates: one spoke per measure, clockwise from the top.
angle <- setNames(pi / 2 - 2 * pi * (seq_along(measures) - 1) / length(measures), names(measures))
polygons <- means |>
  mutate(x = value * cos(angle[measure]), y = value * sin(angle[measure]))
rings <- expand_grid(r = c(0.25, 0.5, 0.75, 1), t = seq(0, 2 * pi, length.out = 121)) |>
  mutate(x = r * cos(t), y = r * sin(t))
spokes <- tibble(measure = measures, x = cos(angle), y = sin(angle))

p <- ggplot(polygons, aes(x = x, y = y)) +
  geom_path(data = rings, aes(group = r), colour = "#d6d6d6", linewidth = 0.3) +
  geom_segment(data = spokes, aes(x = 0, y = 0, xend = x, yend = y), colour = "#d6d6d6", linewidth = 0.3) +
  geom_text(data = spokes, aes(x = 1.12 * x, y = 1.12 * y, label = measure), size = 3, colour = "#535353") +
  geom_polygon(aes(colour = species, fill = species), alpha = 0.12, linewidth = 0.7) +
  annotate("text", x = c(1.6, 1.6, 1.6), y = c(-0.5, -0.7, -0.9),
           label = c("Adelie", "Chinstrap", "Gentoo"), colour = c("#9b2367", "#2e7891", "#c06a1f"),
           hjust = 0, size = 3.2) +
  coord_fixed(xlim = c(-1.35, 2.25), ylim = c(-1.15, 1.15)) +
  labs(title = "Gentoos are the heaviest, with the shallowest bills",
       subtitle = "Species means, each measure rescaled 0–1 over all birds, so no mean reaches the outer ring",
       x = NULL, y = NULL,
       caption = "Data: palmerpenguins (Horst, Hill & Gorman 2020)") +
  theme_lex_viz(grid = "", arrows = "") +
  theme(legend.position = "none", axis.text = element_blank())