Fix not showing tooltip on network graph

Before:
![network_tooltip_before](https://f.cloud.github.com/assets/72138/97817/584fc98a-6709-11e2-8fd8-d6a8acfd582d.png)

After:
![network_tooltip_after](https://f.cloud.github.com/assets/72138/97819/6434bba2-6709-11e2-8e5e-27029692082c.png)