Implicitly stack barY when specified as {x, y} (not {x, y1, y2})

[mbostock]

The current behavior for barY is overlapping bars when specified as {x, y} since this is equivalent to {x, y1 = 0, y2 = y}. If the bars (or areas) are opaque, this can obscure the data. Perhaps we could have implicit stacking by default, such that if the barY is specified as {x, y} (i.e. if y1 and y2 are undefined) then we implicitly apply the stackY transform to create them. And you could always opt-out of stacking by explicitly setting y1 to zero if desired (maybe we could allow y1 to be specified as a constant number, so that y1: 0 works).

[Fil]

I'd say it's fine as it is at this "lower" level, but it should definitely be here by default in a higher-level "Histogram component". Same maybe for a stacked area chart, which should include the seriesX transform (#325) by default.

[Fil]

duplicate of #348, fixed in c78e706