Improve exponential breaks

[teunbrand]

This PR aims to fix #405.

Briefly, it introduces a new breaks function for the exponential transformation: breaks_exp().
It is also incorporated in transform_exp() as the breaks function.

While it is not perfect, it most definitely is an improvement over the old situation. The way I came up with this break algorithm is just by eyeballing it. I found that if the range is greater than 3, the exponential transformation began to deform the space by a lot. When the range is smaller than 3, it wasn't too bad, so we're still using breaks_extended() there. When the range is larger than 3, we're labelling 0 and the last n - 1 integers of the range.

Some examples:

devtools::load_all("~/packages/scales/")
#> ℹ Loading scales

demo_continuous(c(0, 100), transform = "exp")
#> scale_x_continuous(transform = "exp")

Small range reverts to breaks_extended():

demo_continuous(c(0, 2.5), transform = "exp")
#> scale_x_continuous(transform = "exp")

Just after the cutoff point:

demo_continuous(c(0, 3.5), transform = "exp")
#> scale_x_continuous(transform = "exp")

Even though the limits don't include 0, the expanded range does, so we still display 0 here:

demo_continuous(c(100, 104), transform = "exp")
#> scale_x_continuous(transform = "exp")

^{Created on 2024-10-04 with reprex v2.1.1}

[thomasp85]

Thanks for this. We can always improve further if cases with bad results are reported