By default a formula of x on y is converted into a formula of y
on x, while the reverse swap is done only if `backward = TRUE`

.

## Details

This function is meant to be used only as a helper within 'ggplot2'
statistics. Normally together with geometries supporting orientation when
we want to automate the change in orientation based on a user-supplied
formula. Only `x`

and `y`

are changed, and in other respects
the formula is rebuilt copying the environment from `f`

.