Make the border on one bar darker than the others

Question

I have created a bar graph in ggplot2 where 3 bars represent the probability of making 1 of 3 choices.

I want to add a bolded border around the bar that

Answer 1

Similar to Troy's answer, but rather than creating a layer of invisible bars, you can use the size aesthetic and scale_size_manual:

require(ggplot2)
data(diamonds)

diamonds$choose = factor(diamonds$clarity == "SI1")

ggplot(diamonds) + 
  geom_bar(aes(x = clarity, fill=clarity, size=choose), color="black") +
  scale_size_manual(values=c(0.5, 1), guide = "none") +
  facet_wrap(~ cut)

Which produces the following plot: