Format tip pair as a single value when the two values are equal?

[Fil]

By default when formatting pairs (y1, y2), we display ${format(a)}—${format(b)}. It does not feel terribly useful to repeat the value when a and b end up being formatted identically.

If for example you're showing a tip on an interval that is [3.1—3.2] with an integer format, showing "3" (read as "somewhere around 3 something") is as informative and slightly better than "3—3" ("between 3 something and 3 something").

This does not happen often, since hopefully the format discriminates between values, but when it happens we would be as happy with showing ${format(a)} alone.

I tried to think of a case where on the opposite, one might want to show a repetition; for example on a calendar where you want to show an interval that goes "from Sunday to Sunday", as "Sun—Sun". In that case, you would have to create a specific channel since the automatic tip format would now show "Sun" only.

This enhancement would incidentally help with the waffle tips (#2132), but is not necessary for them, and should be discussed on its own merits.

(Note: does not apply when the value is the difference — i.e. hint: {length: true}).

[Fil]

A case found in the wild: the binning of integers between 0 and 7, that results (by default) in small bins such as [4 — 4.5]. If the format was showing only the integer part (as suggested in #1268), this PR would automagically solve this case.

(However in this case, we'd also want the bin labels to be "4" and not "4 inclusive to 5 exclusive".)

[mwaibel-go]

A first step that would allow API consumers to solve this would be to pass the bin boundaries to a custom formatter function instead of each boundary individually.

To be clear, I’m talking about this API:

Plot.binX(
  { y: 'count' },
  { x: 'startTimestamp',
    thresholds: 'day',
    tip: {
      format: {
        x: (date) => getGermanDate(date)
      }
    }
  },
)

Since I’m binning by days, the tip will look like “01.01.2026–01.01.2026”. This could be avoided if I could instead provide the formatter as

format: {
  x: ([date1, date2]) => {
    const formattedDate1 = getGermanDate(date1)
    const formattedDate2 = getGermanDate(date2)
    return formattedDate1 !== formattedDate2 ? `${formattedDate1}–${formattedDate2}`: formattedDate1
  }
}