I meant to follow up and say, that what you're looking for could be added to the existing app. Maybe @kampto would be willing to do it with:
I think the stick in the mud here is do you really want to track averages based on time the device is in the "on" position? I think a more accurate way would be to maybe key in on power usage (e.g., power > 0 is "on" and power = 0 is "off").
