numeric < 1; the fraction of overlap of the conditioning variables
equal_length
if `overlap = 0` non overlaping intervals are produced
all with same length if `equal_length` is `TRUE` (default) or with the same
number of values otherwise.
If `overlap = 0` then `ggplot2::cut_interval` is used to generate
the intervals if `equal_length = TRUE` (default), otherwise
`ggplot2::cut_number` is used. If `overlap` is not zero,
`graphics::co.interval` is called.