Feature request: Display average value, also in selected time interval

[drws]

CPU load and I/O can have great variance so it is sensible to show average values as well. A Windows example would be the average transfer rate line in copy-indication window.

I propose two additions to SysMonTask:

1. Display the average value of the whole timespan that is on the graph. Since it's an overall value it's probably best it is shown under the graph.
2. Additionally provide an ability to freely select a timespan on the graph and dispay the average value in the selected interval. The resulting value could be shown under or in the graph.

Both values (overall average, selected average) should also be displayed with a simple horizontal line in the graph since this also helps to represent the instantaneuous value.