fix date typing in arrow tables

[Fil]

uses the same duck type test as observablehq/inputs#263

addresses observablehq/framework#1376 (for the Plot part)

Note: it's not as good as full Arrow support (where the data would not get transformed into an array of Proxy objects, and valueof would use table.getChild, see #2030).

[mbostock]

We already have Date coercion if the scale type is set to temporal. Can we use that instead here? I.e., change the isTemporal test instead of changing arrayify? I think we want to minimize the logic here.

[Fil]

I don't know if we could do that. As soon as we apply Array.from to the arrow table, the schema information that we need to distinguish between a column of numbers and a column of dates is lost. The data becomes an Array of Proxy objects, with getters that return numbers (even on date-typed columns, per apache/arrow#40892 — note that this change discusses a future improvement to make the Proxy return Date objects again, but I don't think it has happened yet).

But in fact, applying Array.from is already a losing proposition performance-wise, and we should use the columnar format directly (#2030). In a sense, this PR is only a stop-gap measure. I've ported the necessary changes to #2030 too, if we want to take that leap instead.

[mbostock]

I incorporated this into #2115:

plot/src/options.js

Lines 67 to 73 in 4faebbc

	function maybeTypedArrowify(vector, type) {
	return vector == null
	? vector
	: (type === undefined || type === Array) && isArrowDateType(vector.type)
	? coerceDates(vector.toArray())
	: maybeTypedArrayify(vector.toArray(), type);
	}

I think it would be slightly better in the future to avoid materializing the values as Date instances, and instead somehow retain a hint on the column that the numbers represent dates (like Apache Arrow does with vector.type). But that’s “just” an optimization that we can consider in the future.

[Fil]

superseded by #2030