Fix y axis - it shouldn't allow negative times.

Before:

![screen shot 2013-08-30 at 2 48 13 pm](https://f.cloud.github.com/assets/44345/1060928/74d43112-11be-11e3-9132-98d780ad51f7.png)

After:

![screen shot 2013-08-30 at 2 52 38 pm](https://f.cloud.github.com/assets/44345/1060930/878d5f04-11be-11e3-900c-baf9a4b70360.png)