Recently I was asked for an advice of how to plot values with an additional attached condition separating the boxplots. This turns out to be ugly in base graphics, but amazingly simple in ggplot2.

library(ggplot2)

# create fake dataset with additional attributes - sex, sample, and temperature
x <- data.frame(
 values = c(runif(100, min = -2), runif(100), runif(100, max = 2), runif(100)),
 sex = rep(c('M', 'F'), each = 100),
 sample = rep(c('sample_a', 'sample_b'), each = 200),
 temperature = sample(c('15C', '25C', '30C', '42C'), 400, replace = TRUE)
)

# compare different sample populations across various temperatures
ggplot(x, aes(x = sample, y = values, fill = sex)) +
geom_boxplot() +
facet_wrap(~ temperature)