r - How to order bars in faceted ggplot2 bar chart

Question

If I want to order the bars in a ggplot2 barchart from largest to smallest, then I'd usually update the factor levels of the bar category, like so

one_group <- data.frame(
  height   = runif(5),
  category = gl(5, 1)
)

o <- order(one_group$height, decreasing = TRUE)
one_group$category <- factor(one_group$category, levels = one_group$category[o])

p_one_group <- ggplot(one_group, aes(category, height)) +
  geom_bar(stat = "identity")
p_one_group

If have have several groups of barcharts that I'd like in different facets, with each facet having bars ordered from largest to smallest (and different x-axes) then the technique breaks down.

Given some sample data

two_groups <- data.frame(
  height   = runif(10),
  category = gl(5, 2),
  group    = gl(2, 1, 10, labels = letters[1:2])
)

and the plotting code

p_two_groups <- ggplot(two_groups, aes(category, height)) +
  geom_bar(stat = "identity") +
  facet_grid(. ~ group, scales = "free_x")
p_two_groups

what do I need to do to get the bar ordering right?

If it helps, an equivalent problem to solve is: how do I update factor levels after I've done the faceting?

Answer 1

here is a hack:

two_groups <- transform(two_groups, category2 = factor(paste(group, category)))
two_groups <- transform(two_groups, category2 = reorder(category2, rank(height)))

ggplot(two_groups, aes(category2, height)) +
  geom_bar(stat = "identity") +
  facet_grid(. ~ group, scales = "free_x") +
  scale_x_discrete(labels=two_groups$category, breaks=two_groups$category2)

1. make UNIQUE factor variable for all entries (category2)
2. reorder the variable based on the height
3. plot on the variable: aes(x=category2)
4. re-label the axis using original value (category) for the variable (category2) in scale_x_discrete.