Draw convex hull for points in ggplot2

ConvexHull_ggplot2.R

# Script to plot convex hull over points in ggplot2

# Packages
library(dplyr)  # To make code easier to read
library(ggplot2)

# Data Generation
set.seed(1)  # For reproducibility of example

df = data.frame(x = rnorm(100, 10, 5), y = rnorm(100, 10, 5), z = sample(letters[1:5], size =  100, replace = T))

# Plot without convex hulls
p = ggplot(df, aes(x, y, colour = z)) +
  geom_point()

# Computing convex hulls
s = df %>%
  split(df$z)  # Tranform data.frame in lists of data.frames depending on column 'z'

ch = s %>%
  # Compute which points are on the convex hull of each data.frame /!\ Check how 'chull' works
  lapply(., function(el) chull(el$x, el$y))  # 'ch' now contains the row numbers of points on convex hull per sub-data.frame

# Get points for each sub-data.frame using names index
ch = lapply(names(ch), function(x) s[[x]][ch[[x]],]) %>%   
  do.call(rbind, .)  # Join all convex hull points in a single data.frame

# Plot with convex hulls
p.ch = ggplot(df, aes(x, y, colour = z)) +
  geom_point() +
  geom_polygon(data = ch, aes(fill = z), alpha = 0.2)