Tweak vertical range

Hi.

This PR tweak vertical range of graph automated.

## Screenshots
### Series = 2.0, 1.5, 2.5, 1.2
- before
![before1](https://cloud.githubusercontent.com/assets/50920/3561037/4f84e9b4-09a3-11e4-84bf-dc00f4d54eb0.png)

- after
![after1](https://cloud.githubusercontent.com/assets/50920/3561039/5911e482-09a3-11e4-9ea6-1cf8b085b77a.png)

### Series = 20.0, 15.8, 2.5, 2.0
- before
![before2](https://cloud.githubusercontent.com/assets/50920/3561040/5f53b5be-09a3-11e4-8fdf-104ed87a6d0d.png)

- after
![after2](https://cloud.githubusercontent.com/assets/50920/3561041/64fe82e6-09a3-11e4-993f-17f551c9dc27.png)

### Series = 120.0, 200.0, 158.5, 180.0
- before
![before3](https://cloud.githubusercontent.com/assets/50920/3561042/69854f16-09a3-11e4-97ae-cb66b56d0a86.png)

- after
![after3](https://cloud.githubusercontent.com/assets/50920/3561043/6e4a2a8a-09a3-11e4-811b-4549f1b1dbe3.png)

### Series = 20.0, -220.0, 50.5, 80.0
- before
![before4](https://cloud.githubusercontent.com/assets/50920/3561044/7417f672-09a3-11e4-8444-8993f97bea68.png)

- after
![after4](https://cloud.githubusercontent.com/assets/50920/3561046/79bb4746-09a3-11e4-8f05-d7645633732e.png)

If this feature merged, I'm very happy because no tweak required in my app.