Update formatting, add locale support.

[jheer]

This PR updates all axis, legend and template formatting to pass through datalib's formatting support (^1.4.2). As a result Vega/Datalib now control all automatic formatting decisions, with no reliance on (inconsistent) D3 scale tickFormat methods. Most importantly, these facilities support runtime changes to current number and time locale formatting.

For example, to change the number formatting to German (DE-DE):

vg.util.format.numberLocale({
  decimal: ",",
  thousands: ".",
  grouping: [3],
  currency: ["", "\xa0€"]
});

To change the time formatting to German (DE-DE):

vg.util.format.timeLocale({
  dateTime: "%A, der %e. %B %Y, %X",
  date: "%d.%m.%Y",
  time: "%H:%M:%S",
  periods: ["AM", "PM"], // unused
  days: ["Sonntag", "Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag"],
  shortDays: ["So", "Mo", "Di", "Mi", "Do", "Fr", "Sa"],
  months: ["Januar", "Februar", "März", "April", "Mai", "Juni", "Juli", "August", "September", "Oktober", "November", "Dezember"],
  shortMonths: ["Jan", "Feb", "Mrz", "Apr", "Mai", "Jun", "Jul", "Aug", "Sep", "Okt", "Nov", "Dez"]
});

For more about locale settings, see the d3-format and d3-time-format module repositories.

[Fil]

What is the correct method with Vega3, and where is it documented? (Sorry to ask here but it's the only reference I found.)

Inspecting the vega object I found the vega.formatLocale method, and a way to get the result I want — but it seems I have to call it before parsing the spec, is that correct?

[jheer]

Yes, Vega 3 exports the formatLocale and formatTimeLocale methods for localization settings. These are actually direct exports of D3 v4's formatDefaultLocale and timeFormatDefaultLocale methods. These set the global default locales for number and time formatting, respectively.

You will want to invoke these methods prior to instantiating a Vega View and thus before any formatting functions are generated. You should be able to parse a spec ahead of time, but you will need to set the locale before invoking the View constructor for your vis.