Grouping on the x axis in ggplot2

Question

In ggplot I want to create subcategories on the x axis as in:

The solutions presented in Multirow axis labels with nested grouping variables don't work.

Answer 1

Update to version 2.2.0 or higher of ggplot2. Then you can use some more features of the facet_wrap to build the graphic you are looking for. Here is an example:

library(ggplot2)
packageVersion("ggplot2")
# [1] ‘2.2.1’

dat <-
  data.frame(category = c("A", "A", "B", "B", "C", "C"),
             subcat   = c("S1", "S2", "S1", "S2", "S1", "S2"),
             value    = c(73, 57, 7, 23, 51, 87))

ggplot(data = dat) +
  aes(x = subcat, y = value, fill = subcat) +
  geom_bar(stat = "identity", width = 1) +
  geom_text(mapping = aes(label = paste0(value, "%")), vjust = -0.5) +
  facet_wrap( ~ category, strip.position = "bottom", scales = "free_x") +
  theme(panel.spacing = unit(0, "lines"),
        strip.background = element_blank(),
        strip.placement = "outside") +
  xlab("x-axis label")

Answer 2

An example of grouping histograms together with ggplot is provided below (I interpreted the "subcategories" as the values of the factor type. The trick is to use both stat = "identity" and position = "dodge" in geom_bar().

df <- data.frame(class = c("A", "B", "C", "A", "C", "B", "C", "A", "B"), 
                  type = c("alpha", "beta", "gamma", "gamma", "beta", "gamma", "alpha", "beta", "alpha"), 
                  value = c(100, 200, 300, 50, 60, 120, 400, 300, 160))

ggplot(df, aes(class, value, fill = type)) + geom_bar(stat = "identity", position = "dodge")