The dense interval option on the area mark seems to require the y option

[mbostock]

I expect this to work:

Plot.areaY(data, {x: "date", reduce: "count", interval: "day"}).plot()

After all, it works for a line:

Plot.lineY(data, {x: "date", reduce: "count", interval: "day"}).plot()

Instead it seems you have to set the y option to anything:

Plot.areaY(data, {x: "date", y: true, reduce: "count", interval: "day"}).plot()