[Bars] Support for stacking datasets

Adds `stacked` to the plot options, set to `false` by default. When set to `true`, datasets are stacked on top of one another, rather than interleaved alongside each other.

![captura de pantalla 2013-07-09 a la s 13 03 51](https://f.cloud.github.com/assets/210225/769947/f5c5d2d8-e8c1-11e2-831c-11ff046e0049.png)