Allow custom formatters for hoverformat and tickformat

[ianrcooke]

I'd like to be able to provide a callback to hoverformat and tickformat so that the data can be clearly separated from its presentation and I have finer-grained control than the mini-languages provided by D3.

The issue I'm trying to solve is showing data values over time where time is expressed in relative milliseconds. I want to have x values (time) accessible, e.g. in click events, in their original form (a millisecond count) while displaying the data in hovers and on the x axis ticks as hh:mm:ss.milliseconds. Currently I appear to have to convert the milliseconds to datetime strings and then when capturing a click event do the conversion back from a datetime string to milliseconds to get my original data.

Other Javascript charting libraries like Chart.js and Highcharts allow this sort of control of display without modification of the underlying data.

A feature like this would also address issues like: #1744

[etpinard]

Duplicate of #1464