As a user, I want an option to view breakdown charts as percentages instead of quantities

From @gpeal:

![image](https://user-images.githubusercontent.com/50083/54859741-9c452400-4cd6-11e9-934e-1221ae943997.png)

> This graph would be really interesting to see with each bucket as a %. Might be easier to show any improvements since we added pull reminders

