Fix legendHeight when rendering secondYAxis (0.9.x)

We need to divide `numberOfLines` by `columns` when calculating our `lineHeight` for graphs using `secondYAxis`.

Before:
![render](https://f.cloud.github.com/assets/494338/334110/f9d00b02-9c6b-11e2-9f7a-a45c0bfaccd6.png)

After:
![render2](https://f.cloud.github.com/assets/494338/334112/fd6f9598-9c6b-11e2-9d29-c6baa05d8c40.png)

See #253 for discussion.