Fix category scale Zoom/Pan proportion problems

[subogero]

Plus panning on indexScale when zoomed in also causes zoom.

Zoom: the zoom parameter is a proportion. On linearScale it does work like that, but on the indexScale it's adding/removing 1 label at a time. Totally broken with large datasets. Make it work like linearScale.

Pan: calculate dIndex first, limit if 0 .. lastLabelIndex range would be exceeded, then modify minIndex, maxIndex with the exact same dIndex.

Pan on indexScale currently unusable with large datasets. Make pan distance proportional to mouse/touch gesture size.

Probably fixes some other related issues.

Pull request coming immediately.

[subogero]

Also see https://www.reddit.com/r/webdev/comments/8nqcq0/best_js_lib_for_charts/

[benmccann]

Also reported in #190

[benmccann]

Pending PR: #152

[kurkle]

I believe this was fixed in #476