Fix boxplot

Depends on #929.
Some bug fixes over the boxplot chart generated at the scipy sprint. Now it is using the categorical axis in a proper way (instead of an hybrid thing we had before at the time of drawing the segments... it was used some "kind" of linear x values instead of the categories in the categorical axis). Also changed quad for rect, deleted unused things and improve the styling a little bit... now look in this way:

![boxplot](https://cloud.githubusercontent.com/assets/1640669/3768303/11d99bb8-18d6-11e4-8275-62fa411db432.png)

(forget the "cutted" label... it is another issue)
OK, using multiples data sources is a nice idea I completely forget about... because up to now all the charts available (except the boxplot) works OK with only one data source.
I think your request is acceptable and interesting... I will change the label of this PR to WIP and begin refactoring this thing in the way you proposed. Thanks for the feedback.