Format legend value to billion, million or thousand

Before:
![screenshot - 271114 - 21 45 59](https://cloud.githubusercontent.com/assets/581607/5221593/d5d6e1dc-767e-11e4-831d-9dc4ce61152e.png)


After:
![screenshot - 271114 - 21 47 49](https://cloud.githubusercontent.com/assets/581607/5221600/1295a950-767f-11e4-8ca0-085c9d108b2d.png)

Values are rounded up, but a possible improvement could be to show one or two decimals for amounts with less than two digits after formatting (example 1,4B instead of 1B or 2,8B instead of 3B)