Prevent tooltip flicker

While working on #1993 I noticed this flicker in the tooltip that really bugged:

![2014-11-20 14_05_41](https://cloud.githubusercontent.com/assets/1329312/5132997/969460aa-70be-11e4-9e06-111c973ccd7e.gif)

It happened because the default position for the tooltip is to the bottom left of the cursor. This is outside of the charts container when entering from the bottom though, hence the flicker.

Now, when the vis chooses a direction for the vis it will keep that direction until it is no longer valid.
![2014-11-20 14_11_54](https://cloud.githubusercontent.com/assets/1329312/5133069/32ee6e82-70bf-11e4-90dc-b5a9c89b70f4.gif)