Allow expressions in facet formulas

[dgrtwo]

I'd like to be able to put expressions in faceted formulae instead of just column names.

For example, I'd like to be able to facet by a "cut" of a column:

ggplot(mtcars, aes(qsec, mpg)) +
  geom_point() +
  facet_wrap(~ cut(wt, c(0, 2, 4, 6)))

rather than needing to do something like:

mtcars$cut <- cut(mtcars$wt, c(0, 2, 4, 6))

ggplot(mtcars, aes(qsec, mpg)) +
  geom_point() +
  facet_wrap(~ cut)

[DanRuderman]

Great feature. Syntax-wise, how would you handle the overloading of + as both an arithmetic operation and a separator in the formula?

[hadley]

It would have to be handled the same way as modelling formulas — if you want addition you need to surround in I()

[DanRuderman]

First attempt in #1722. Will be shelved until some infrastructure changes take place.

[thomasp85]

#1722 has now been ported to the new facet implementation in #1633