only default to bar for x or y is zeroed

[mbostock]

Fixes #1272.

[Fil]

lgtm!

BTW I'm not sure #1238 has documented the reducer's zero option? I can't see it.

[mbostock]

I think this is better, but I think we could still refine this logic. Like, it makes sense to me that this is a line:

Plot.auto(weather, {x: "date", y: {value: "temp_max", reduce: "mean"}}).plot()

But I’m not convinced that we should switch to a bar when you tell it that zero is meaningful:

Plot.auto(weather, {x: "date", y: {value: "temp_max", reduce: "mean", zero: true}}).plot()

I think a line + zero rule would be equally appropriate (and less arbitrarily different):

Plot.auto(weather, {x: "date", y: {value: "temp_max", reduce: "mean", zero: true}, mark: "line"}).plot()

Or perhaps even an area:

Plot.auto(weather, {x: "date", y: {value: "temp_max", reduce: "mean"}, mark: "area"}).plot()

On the other hand, I think it would be surprising if histograms rendered as lines or areas by default, but I can’t articulate why.

Plot.auto(weather, {x: "temp_max", mark: "line"}).plot()