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)
Btw, this is fact pertinent to the Big refactor branch... it is bad that Github does allow to review against another branch... because you end up reviewing things twice... maybe the solution is make PRs against another PRs but it is complex because you will end up with a giant PR, and all the child PR would need to be reviewed before merging the father one... so... more pain...