bins argument of geom_histogram and stat_bin argument off by 1

[alistaire47]

When looking at this SO question, I realized that when I set the bins argument of geom_histogram or stat_bin, the resulting histogram has one more bin than the argument. For example:

ggplot(data = iris, aes(x = Sepal.Length)) + stat_bin(bins = 3)

produces

which certainly doesn't seem like what most people would expect. Is this a zero-based index issue?

Apologies if I'm missing something obvious, and thanks for all your work.