descending order

[mbostock]

Instead of interpreting reverse: true as “sort by descending natural order”, the ChannelDomainOptions interface now has an order option which can be ascending, descending, or a comparator function (which compares two [name, value] tuples). This better matches the SortOrder interface, and enables two new features:

1. If you set order: null you get input order (order of first occurrence in the data).
2. If you set reverse: true you get true reverse order, putting null/unorderable values first when sorting, or reverse input order when not sorting, in the same manner as the basic sort transform.

I’m leaving this as draft because we should probably add tests for both of these new features. Added tests!

Fixes #1460. Ref. #1606 (comment)

[mbostock]

I’m following the principle here that test names should indicate which features are being tested, rather than tests being feature-agnostic examples. We can have the latter, too, but here the tests are intended to test imputed channel domains so it’s better to call them that.

[Fil]

As an aside, we can now do this if we need to use the raw grouped values for sorting (not that I'd recommend it, but…)

  channels: {index: (d, i) => i},
  sort: {
    fy: "index",
    reduce: (index) => index,
    order: ([group, index]) => console.warn(Array.from(index, (i) => movies[i])) // replace with a comparator on ([groupA, indexA], [groupB, indexB])
  }