We have a requirement to use custom tick format at some zoom level. For example:
I want to add "Year" postfix for years but keep default formatting for other zoom levels.

I prepared a PR to add possibility to pass not only string but also a configuration object to axis tickformat. Since it is my first PR I did it in my fork as per the guideline
apalchys#1

@rreusser / @etpinard could you please review?