Create areas with upper/lower domain

I've added the possibility to create an area with a upper/lower domain. The lower value of the existing area is always the same for all the serie. Now you can define a lower value for the area for each point:

![ae551a6a-3369-11e3-824b-dc80074f58c5](https://f.cloud.github.com/assets/5390887/1323922/85b4c83a-349f-11e3-9597-64d14f61e842.png)

To activate this option only need to pass a 2-value array to the y value of the area:

sin.push({x: i, y: i % 10 == 5 ? null : [Math.sin(i/10),Math.sin(i/10)/2]});

That don't affect to the existing line or area. An area with a single y value is represented as usual with a same lower value for all the area (normally 0 base).