Fixing container for tooltips to avoid overlap.

I forgot to add this change yesterday:

![before](https://s3.amazonaws.com/f.cl.ly/items/0c081M012o2Z361h3411/Screen%20Shot%202013-07-09%20at%2010.37.35%20AM.png)

Now the tooltip lives outside the `sparkline` `div` so it isn't cut off.
![fix](https://s3.amazonaws.com/f.cl.ly/items/1B2X072n2y2J18401H1Y/Screen%20Shot%202013-07-09%20at%2010.37.08%20AM.png)