Plot convex hulls according to a factor

ggplot_chulls.R

require(ggplot2)
require(plyr)

# make toy data
set.seed(1)
dataf <- data.frame(
  x = rnorm(100),
  y = c(rnorm(50,-1), rnorm(50,1)),
  cat = rep(c("A","B"), each=50)
  )

ggplot(data=dataf, aes(x=x, y=y, color=cat)) + geom_point()

# get convex hull coordinates
# chull function gives you which points delimit the minimum convex hull

chulls <- ddply(dataf, .(cat), function(df) df[chull(df$x, df$y), ])

# plot polygons

ggplot(data=dataf, aes(x=x, y=y, color=cat)) + geom_point() +
  geom_polygon(data=chulls, aes(x=x, y=y, group=cat), fill=NA)

# if you wanna get fancy with the colors
# (will give annoying warnings if number of groups is smaller than 3, ignore)

ggplot(data=dataf, aes(x=x, y=y, color=cat)) + geom_point() +
  geom_polygon(data=chulls, aes(x=x, y=y, fill=cat), alpha=0.2) +
  scale_color_brewer(palette='Set1') +
  scale_fill_brewer(palette='Set1')

For those coming here for the code, here's the plot produced by the code above