Add previous timestamp to tickFormatter

[jamesarosen]

I'd love to be able to have my tickFormatter return more concise information for most points, but more detailed information when relevant.

For example:

|         |         |         |         |         |        |
27        28        29        30      May 1       2        3

Or:

  |         |         |         |         |         |        |
22:00     23:00    March 13   01:00     02:00     03:00    04:00

The only way to know to add the May or to replace 00:00 with March 13 is if I can tell that the current tick has crossed some threshold from the previous tick.

Thus, I propose:

tickFormatter: (time: Time, type: TickMarkType, locale: string, previousTime?: Time) => string

previousTime would be undefined for the first (leftmost) tick.

[jamesarosen]

This is more-or-less possible with the existing TickMarkType argument.

function tickMarkFormatter(seconds, tickMarkType) {
  const date = new Date(seconds * 1000)
  if (tickMarkType === TickMarkType.DayOfMonth) {
    return new Intl.DateTimeFormat('default', { day: '2-digit', }).format(date)
  } else if (tickMarkType === TickMarkType.Month) {
    return new Intl.DateTimeFormat('default', { month: 'short', day: '2-digit' }).format(date)
  }
}

produces

|         |         |         |         |         |        |
27        28        31     Apr 01       02        03       04

But if the user is zoomed in to a particular day, all the tick marks get TickMarkType.Time, so the date never appears:

  |         |         |         |         |         |  
14:00     15:00     16:00     17:00     18:00     19:00

(And similarly for days in the middle of a month.) I think regardless of the zoom level, there should always be at least two different TickMarkTypes represented for context.