implicit zero

[mbostock]

Fixes #2331. Breaks a bunch of tests, of course. And should probably only apply to positional scales, not to (e.g.) color? And having an implicit rule would also be nice. Is this too opinionated?

[Fil]

I don't mind it being applied to color as well, in the linear case. But it's probably less important.

Adding a default rule seems a tad too opinionated. The new visible tick at 0 is enough to ground the chart.