Minipiechart

this creates a mini pie chart attached to the selection bar that shows each series' percent of the total for that x-value: 
![screen shot 2014-11-29 at 12 40 12 pm](https://cloud.githubusercontent.com/assets/4649503/5235729/0c5540a4-77c5-11e4-8761-5695dea72a63.png)

not-quite-ready-for-prime-time, as when you move a series to a new chart, the pie chart doesn't follow the series, but i'm afraid fixing that is beyond my current knowhow.  thought it could be a neat feature.