fix: Center period labels over their own period when using ticklabelindex

[my-tien]

Fixes #7876 by rewriting the way period label positioning works when using ticklabelindex.

Previously, the next labeled tick was used as the period end for the previous tick. This was problematic when using ticklabelindex, because sometimes the next labeled period is not necessarily adjacent to the last labeled period.
Now positionPeriodTicks can accept an array of periodEndTicks instead.

[camdecoster]

Most of this works, but there are a few changes that need to be made. I'm happy to talk through how to do some of this.

[camdecoster]

Instead of saving an array of end ticks for each tick, what if we save the end tick to each tick? Then you could check for an end tick on each tick and use a different code path. You'd also be able to get rid of the new arg in positionPeriodTicks.

[my-tien]

Let me know if I didn't implement it as you intended.

[camdecoster]

What should happen if there's a major tick that doesn't coincide with a minor tick? Should that also be included?

[my-tien]

No, when ticklabelindex is used, only minor ticks should labeled. This step is just to handle the special case when a minor tick coincides with a major tick in which case the major tick should be labeled instead (because the minor could be removed later).

[camdecoster]

I understand why you're using length -1, but that means that you're going to lose the last label. You could run through the entire array and check that largerTicks[labelIndex + 1] is defined before pushing it. This logic would also need to change if you start adding the end tick directly to the tick itself.

[my-tien]

Not entirely sure what you mean here. From my understanding, if you reach largerTicks.length -1, you are done because you are at the end of labelTickValsAscending and an additional label would overflow.