Auto mark: zero baseline default for rules

[tophtucker]

Mike writes:

The mark-based zero check is also forgetting about rule. This should have a y = 0 rule like rect does.

Now it does!

Before	After

Plot.auto(olympians, {x: "date_of_birth", y: {value: "height", reduce: "mean"}, mark: "rule"}).plot()

A couple notes:

barY or areaY or rectY or… ruleX? Yes; ruleY would be the one parallel to the y baseline, and can make sense without any zero baseline; ruleX is the one orthogonal to the baseline, which has to “stand on it” (like barY).

Merely adding || mark === ruleY and || mark === ruleX to the conditional breaks autoNullReduceContinuous and autoNullReduceDate, so I also added the X && and Y && checks, which seem like a good idea anyway.

Broken	Fixed

The new test case demonstrates the existing issue that a rule can overhang its baseline… but that sounds like a whole can of worms and I wouldn't worry about it lol:

[mbostock]

Nice. 👍