Add comparator function to ordinal domain sort

[mhkeller]

See #151

[mhkeller]

This also adds a fix where unique dates would not be calculated properly. Now, it compares the dates using .getTime() instead of relying on Set to calculate uniqueness.

[mattlangeman]

Looks good! I think the two instances of set.add(val) that are outside of the if are not necessary, but otherwise this should fix the odd sort behaviour for dates.

[mhkeller]

Great thanks for the help in spotting that! I forgot to delete that line from the prior implementation.

[techniq]

Thanks @mhkeller so getting to this so quickly. Looks great

[mattlangeman]

My other thought would be to tweak the comments to say something like

* Date objects of the same date are not equivalent in a Set
* If val[k] is a date we turn it into a time value with getTime, otherwise getTime returns val[k].
* Push val[k] to results, though, so that we aren't converting the users data.

It took me a bit longer than it should have to understand the if block because I was expecting it to be checking if val[k] was date.

[techniq]

FYI, D3 does something similar with it's InternMap / InternSet (used for group/rollup/etc). Could we use those here as well?

[mhkeller]

@techniq yea that's also a good idea. Another helpful d3-array function that slipped my mind!

[mhkeller]

Thanks you both spotting this and your very helpful comments today!

[techniq]

Thanks @mhkeller for the quick fix and awesome library!

[mattlangeman]

Thanks @mhkeller for the quick fix and thanks @techniq for pointing out the helpful functionality available from d3.