Fix scaled Y axis drawing with vertical graph padding

Modifies Axis.Y.Scaled to render correctly in cases where the owning graph has a non [0, 1] domain (e.g. with significant top or bottom padding).

I wasn't sure how to make a unit test to demonstrate this issue, since the effect is mainly visual.  To easily verify the incorrect behavior, the graph in examples/scaled.html can be modified to have a large padding.top, padding.bottom, or both.  Then when the graph is rendered, hover over some points and verify that the y axis labels and ticks do not correspond to the graphed values.

*Before*
![before](https://f.cloud.github.com/assets/1253708/1337811/e5fbe952-35db-11e3-9c30-8ca693818a07.png)

*After*
![after](https://f.cloud.github.com/assets/1253708/1337813/eb2ab638-35db-11e3-8d82-6e25c2d25458.png)