Feature request: exp scale

[alephpi]

First thanks for your awesome work, it really solves many low-level headaches for us.

In my star map project, I try to visualize the star brightness by the point radius. Given star magnitude in data, since the magnitude is log-scale to the brightness, I need exp scale for such conversion.

Related to #48

BTW, is it possible to have arbitrary scale by providing a scaling function? I think that will be much convenient for customizing scales.