a box-esque plot with a greyscale gradient showing the density of data

gradient_box_plot.R

# box-ish plot with colour gradient giving smoothed densities

# some fake values
x <- c(rnorm(150, -0.5,  1.3),
       rnorm(30, -1,  0.8),
       rnorm(20, 0.1,  0.6))

# how smooth the gradient is
n_levels <- 100

# kernel density estimate, to smooth out the gradient
d <- density(x, n = n_levels, adjust = 2)
dens <- d$y / max(d$y)
locs <- d$x

# boundaries for plotting
lower <- locs[-length(locs)]
upper <- locs[-1]

# shades for plotting
levels <- 0.95 - dens / 2
cols <- grey(levels)

# assume it's on an empty boxplot
plot.new()
boxplot(x, lty = 0, cex = 0, ylim = range(d$x) * 1.2, las = 2)

# plot all segments of density
for (i in seq_len(n_levels)) {
  rect(0.8, lower[i],
       1.2, upper[i],
       col = cols[i],
       lty = 0)
}

# a border around the whole thing
rect(0.8, min(lower),
     1.2, max(upper),
     border = grey(0.8))