[StackedPlot] - Stacking is done on each valid project call

Closes #897 
I wonder if maybe we shouldn't stack at all if we don't have a projector? But it's not being rendered so I suppose it's fine.

Also, I think we should avoid `isNaN`, as `isNaN(undefined)` returns **true**. Use something like
```Javascript
function isReallyNaN(v) { return v !== v; }
```
instead.