Filter graphs that does not change

[renchap]

We should be able to hide :

• graphs that have all values at 0
• graphs that have all values the same on the timeframe

Improvement : add a slider to choose a percentage threshold, and hide graphs if all their values change less than the threshold.

This would be awesome to be able to quickly see only "active graphs". For example, this will allow to hide all network switches ports without trafic.

Note: if real time graphs are implemented before, we must refresh the graph state.