Plot.link default channels?

[Fil]

It might be nice to set "x1", "x2", "y1", "y2" as the default channels for Plot.link.

This would allow to write:

Plot.link([{x1: 0, y1: 0, x2: 8, y2: 8}])
instead of
Plot.link([null], {x1:[0], y1: [0], x2: [8], y2: [8]})
for the common case of using links as support lines.

Also, it might make an interesting combination with the stackY plugin (#109), which results in y1-y2 values. (no, it doesn't change anything on that front)

[mbostock]

I think we should discuss default channels more holistically. I haven’t formed an opinion yet of what we should do here.

[mbostock]

I feel like we shouldn’t do anything here. As I argued in #176 (comment) I think columnar channels are the preferred representation, so I think it makes sense to favor specifying channels rather than having (too many/more) default accessors. It’s a bit of a gray area though and I might change my mind.