Change graph definitions so that time frames are per graph, not per metric

[pcockwell]

Don't allow metric A to be -4 to 0 while metric B is -8 to 0 (on the same graph). Should make it so that each graph has a time frame, and all metrics in that graph are given that time frame.

You should assign this to me if possible. Either way, I'll take care of it - just making a note.