fixes the issue #13 "Turning off a column disables the total at the bottom"

Frankly, it's not very elegant solution, but it's simple and it's works:

![2014-11-13 23-36-40 kerbal space program](https://cloud.githubusercontent.com/assets/6450733/5036862/4fcd9954-6b94-11e4-8807-b4ac087c68b6.png)