Linechart area rendered at the zero line

Add an option in lineChart that allows drawing the area at the zero line instead from the bottom of the chart. Suitable for linecharts with negative values.
Before:
![a2](https://f.cloud.github.com/assets/952085/121807/207be188-6dfd-11e2-8efb-921d22423028.png)
After:
![a1](https://f.cloud.github.com/assets/952085/121808/2450062c-6dfd-11e2-95fc-14061df19c62.png)