consistent intervals with interval.every

[mbostock]

Fixes #63. Fixes #64. We need to finish the documentation, and decide whether it is acceptable that d3.timeHour.every(…) is no longer aligned at midnight.

TODO

• Update documentation (document d3-time#66 (consistent intervals with interval.every) d3#3795)

[Fil]

Seems like a net positive — the current intervals are v. surprising when not strict divisors of 24 —, but indeed a breaking change.

Playing with this in this notebook I can see how it could be surprising for users who expect 2, 3, 4, 6 and 12-hours periods to align with midnight; I've tried to add an “align” option (we could add this as a hint in the documentation or release notes). Putting in a DST change to make things more difficult :)

[mbostock]

I don’t think it’s worth the trouble to consider this a major version bump across D3. The current behavior is broken in many cases. In the case of UTC ticks, the behavior is essentially unchanged. And for local time ticks, the only difference you can see is the two-days ticks are now uniformly spaced rather than resetting on odd-numbered months.

Before:

After:

The downside is that there’s no guarantee now that you will see the “February” because Feb 1 can get skipped in favor of Feb 2. But the default time ticks in D3 are already so much more confusing than Plot’s approach. I think this is a net improvement.

Notebook: https://observablehq.com/d/8c469f7949f95a23

[mbostock]

Also, we already changed the visible behavior of utcTicks by adopting unixDay instead of utcDay in #58, which is why this PR doesn’t change the visible behavior of utcTicks; if we did the same thing for local time as described in #62 then this PR wouldn’t change the visible behavior of timeTicks, either.

Of course, it is a difference that we are talking about not just changing the behavior of utcTicks/timeTicks but the intervals themselves, but it’s also confusing to have two versions of every interval, and for utcTicks/timeTicks to use the newer (and less well-known) intervals to avoid the surprising behavior, while the existing (and most well-known) intervals still behave surprisingly.

[Fil]

lgtm!