Fix stacked axis ranges

Stacked axis ranges are currently defaulting to 1, or 100% of their max value. Stacked graphs, by virtue of being stacked, will always exceed 1. Set the max to that value, the ratio of the highest value in the series.
don't know if you care, could shorten it to
```javascript
rsGraph = scaleId ? yScales[scaleId](graphMax) : rsGraph.renderer.domain().y[1];
```
... but i don't see any reason to turn a simple `if`/`else` statement into something harder to read.