Example of drawing convex hulls around grouped points in R using dplyr and ggplot.

convex_hulls.png

convex_hulls_facetted.png

ggplot_convex_hull.R

# This is adapted from a nice example at:
#   http://f.briatte.org/r/convex-hulls-with-dplyr-and-ggplot2
#   https://gist.github.com/briatte/50cb31531966c99a9acc
#
library(ggplot2)
library(dplyr)

# Parameters for test data and graph
Ngroups <- 4
GroupSize <- sample(4:20, Ngroups, replace = TRUE)
Facets <- TRUE

# Convex hull function. Takes a data frame with:
#
#   - x,y columns
#   - an optional group column 
#   - any additional columns
#
# Returns a new data frame with rows reduced to those
# for convex hull vertices and a new hull column giving
# the order in which to plot (e.g. with geom_polygon).
#
hullfn <- function(dat, 
                   xcol = "x", ycol = "y", 
                   groupcol = NULL,
                   hullcol = "hull") {

  do_hull <- function(xs, ys) {
    ii <- 1:length(xs)
    hi <- chull(xs, ys)
    match(ii, hi)
  }
  
  cols <- colnames(dat)
  ix <- match(xcol, cols)
  iy <- match(ycol, cols)
  colnames(dat)[c(ix, iy)] <- c("x", "y")
  
  grps <- !is.null(groupcol)
  if (grps) {
    dat <- group_by_(dat, .dots = groupcol)
  }
  
  dat <- dat %>%
    mutate(hull = do_hull(x, y)) %>%
    arrange(hull) %>%
    filter(!is.na(hull))
  
  if (grps) dat <- ungroup(dat)
  
  # restore original column names
  colnames(dat) <- c(cols, hullcol)
  
  dat
}

# Generate some test data
xy <- data_frame(
  igrp = rep(1:Ngroups, GroupSize),
  grp = LETTERS[igrp]) %>%
  
  group_by(grp) %>%
  
  mutate(x = runif(n(), igrp/2, igrp/2 + 1),
         y = runif(n(), 0, 1)) %>%
  
  ungroup()
  
# Get data for convex hulls
hulls <- hullfn(xy, groupcol = "grp")

# Plot points and convex hulls
g <- ggplot(data = xy, aes(x, y, colour = grp, fill = grp)) +
  geom_polygon(data = hulls, alpha = 0.4) +
  geom_point() +
  guides(colour = FALSE, fill = FALSE) +
  coord_equal() + 
  theme_bw() +
  theme(axis.text = element_blank(),
        axis.title = element_blank(),
        axis.ticks = element_blank(),
        panel.grid = element_blank())

if (Facets) {
  g <- g + 
    facet_wrap(~ grp, scales = "free") +
    theme(strip.background = element_blank())
}

print(g)