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)
This confuses me a little too. Aren't we in the middle of constructing all these `_list` variables in this loop? Should't anything that sets/gets/does anything with the entire list occur after the list, after the list is constructed? Or alternatively, depend on the loop index somehow? It does not make sense to me at first glance to have an operation involving a not-fully-constructed list inside the loop while it is still being "filled out".