added shared crosshair to dashboards to track better all graphs

HI @torkelo 

I've added ( as promised yesterday) a shared crosshair mode to track better all graph behaviours.

![image](https://cloud.githubusercontent.com/assets/5883405/4453381/bcab4b28-485a-11e4-8536-a3070769bdce.png)


This is the result.

![image](https://cloud.githubusercontent.com/assets/5883405/4453366/7f9224a0-485a-11e4-942d-5448cafff455.png)

I hope you like this new PR!!