An implicit group transform for z-less areas?

The areaY mark gets an implicit stack transform that works well when there’s a z channel:

Plot.areaY(industries, {x: "date", y: "unemployed", z: "industry"}).plot()

But when there isn’t a z channel, stacking doesn’t really help, because you’re still drawing a single shape for the entire dataset, resulting in zigzags:

Plot.areaY(industries, {x: "date", y: "unemployed"}).plot()

This is a little more visible when drawing a line:

Plot.lineY(industries, {x: "date", y: "unemployed"}).plot()

If you want to draw a single shape, you need the group transform:

Plot.areaY(industries, Plot.groupX({y: "sum"}, {x: "date", y: "unemployed"})).plot()

Maybe areaX and areaY could apply a group transform by default (using groupY and groupX respectively) when no fill, stroke, or z channel is present?

Start by comparing the areaY and areaX mark behavior with the existing Plot.groupX and Plot.groupY examples in the issue. Determine how the absence of fill, stroke, and z should affect the implicit transform, then verify the area and line examples produce a single shape without changing grouped cases.