Support 'symlog' scale

[wass88]

Matplotlib supports 'symlog' scale.
https://matplotlib.org/examples/scales/scales.html

Symlog is defined as the following expression.

(-1 < x < 1) ? x :
(x >= 1) ? log(x) :
  -log(-x)

This scale is useful for extensive domains.

Is it possible to support symlog in d3-scale? Otherwise, is there a workaround to use custom scales?

[mbostock]

Seems like a reasonable suggestion.

[danielnaab]

@mbostock If I were to look into this - is there a preferred implementation direction? One could add an option to the log scale, or create a separate symlog module.

If a separate module, I'm curious if there is a good pattern to follow to extend the log scale without duplicating code.

Any suggestions would be greatly appreciated!

[slavikdenis]

Looks great so far :) Would be really great if it would work with generating axis and rescaling while zoom and pan! Keep up the good work @danielnaab

[mattpitkin]

It would be great to see the symlog scale (in part so it can be included in Vega vega/vega#1392, Vega-Lite vega/vega-lite#4127, and Altair vega/altair#1077) and it looks like @danielnaab has done nice work on it. Is there likely to be movement on merging #134 soon?

[jkreft-usgs]

@danielnaab just added documentation as requested by @curran

[mbostock]

I have an implementation in #156 slated for release in d3-scale 2.2.0. I ended up using a slightly different formulation of symlog, though, as recommended in “A bi-symmetric log transformation for wide-range data” by Webber, 2012.