Add a chart showing per-contributor points breakdowns

[thehale]

It would be really nice to have a chart that showed the relative amount of work put in by each contributor over the course of a sprint. This feature can start with a simple chart showing total points completed per contributor (the blue line), and future versions can support showing additional bars (stacked even?) for different types of tasks.

It would also be great to support splitting points between multiple people who contributed to a single task, but that doesn't have to come immediately.