Expose tickFormat?

[jheer]

The tickFormat method is a useful utility, but currently only invokable via a scale instance. It would be helpful to also expose it as part of the d3-scale API - we would immediately make use of this in Vega!

[mbostock]

You mean something like d3.tickFormat(domain, count = 10, specifier)?

Or perhaps d3.tickFormat(start, stop, count = 10, specifier) to be closer to d3.tickStep?

[jheer]

I was thinking of using the method as-is in https://github.com/d3/d3-scale/blob/master/src/tickFormat.js: d3.tickFormat(domain, count = 10, specifier)

That said, either would meet our needs well. Thanks!

[mbostock]

Added to #156.