MRG: matplotlib example and error bar colour

Which one is ROOT?

![matplotlib](https://f.cloud.github.com/assets/202816/183678/9d0c0cf6-7cac-11e2-84d6-81a7a8a6b6ec.png)

![ROOT](https://f.cloud.github.com/assets/202816/183679/9eec1f48-7cac-11e2-8f14-a01c3552efdc.png)