singlepoint charts have the y axis's domain reversed

I created a linechart that is actually a constant. NVD3 interprets it as singlepoint. The y axis is wrong in this case, the generated +-1% domain is increasing top to bottom (ie: 99%, 100%, 101%) instead of decreasing like normal.

![Screenshot from 2013-02-03 21:29:36](https://f.cloud.github.com/assets/1908720/122301/d34ec31c-6e47-11e2-831a-c3c14af68fb4.png)

This patch fixes that.