Interpreting Graphs

I could be wrong, but the way it was explained to me is that it's measure your CPU usage at the time for the entire host machine. That is, if you use enough CPU power to warrant all four processors, the graph will show 400%.

In Linux, the usage percentages of all CPUs are summed. Ergo, with 4 processors, at full usage the percentage is 400%.

If, say, the percentage on your graph is 20%, that could mean anything from one CPU is at 20% usage while the other three are idle, or all four CPUs are at 5% usage.