Have AreaPlot add a padding exception at y=0.

When setting the baseline for other plots, I needed a pattern like
```
if (this.yScale instanceof Abstract.QuantitiveScale) {
  var scale = <Abstract.QuantitiveScale> this.yScale;
  ...
}
```
So is there a reason you don't need the check here? Is `yScale` guaranteed to be an `Abstract.QuantitiveScale`?