Allow date strings when using time intervals

[yurivish]

I came across the behavior shown in this notebook, where plotting date strings on a utc scale ceases to work if the scale additionally has an interval property.

Plot.dotX(dates).plot({ x: { type: "utc" } }) // dots are rendered

Plot.dotX(dates).plot({ x: { type: "utc", interval: d3.utcDay } }) // dots are not rendered

According to the docs time scales coerce channel values to dates, so I think the first chart is right, and there might be something weird happening when the interval option gets involved.

[mbostock]

Temporal scales apply date coercion for channel values here:

plot/src/scales.js

Lines 245 to 248 in 2b318ac

	case "utc":
	case "time":
	options = coerceType(channels, options, coerceDates);
	break;

This happens after the interval transform is applied here:

plot/src/plot.js

Line 121 in 2b318ac

applyScaleTransforms(channels, options);

And the interval you specified (d3.utcDay) doesn’t support promoting date strings to dates, so you get invalid dates.

d3.utcDay("2022-11-12T00:00:00.000Z") // Invalid Date

I don’t think we should move the coercion of scale channel values to before the scale’s transform; we only want to coerce the values that will be directly used by the scale, and let the scale transform do whatever it wants for maximal flexibility.

So, the fix here would need to be changing d3.utcDay to allow date strings as input, or for Plot to provide its own date intervals (e.g., interval: "day") that wrap D3 time intervals and do the same.