From what I understood;
Lets say Player [A] is playing on a regular basis, and is more active at certain moments of the day than he is at other moments, due to things such as work, sleep and going out with friends. Lets say he plays between 6-8AM at home, checks at work every now and then between 8AM-5PM, plays back at home actively again between 5PM-9PM, and goes out with friends after that, and doesn't check back in untill 6AM.
Now, grepointel will keep track of this; Lets say that when Player [A] is at work, he makes less actions because he is occupied with his tasks. The online weighting will set off his activity in the morning towards his activity when at work; If between 6AM-8AM the online weighting shows '120', and between 8AM-5PM shows '30', that means for every 4 (120/30 =4 ) actions Player [A] makes in the morning when at home, he only makes 1 action while at work. From this, you can derive his activity; even if there is an activity bar at every hour of the day, the online weighting gives an indication of the moments at which there are relatively fewer actions made by Player [A].
If this is wrong, feel free to correct me - this is what I've been using, and it's been correct so far.
PS: Grepointel is in no way an online indicator; just because a bar shows 'low activity' at a certain point of the day, doesn't mean that player is never online at that very moment. My grepointel shows I'm never online at 3PM, but I'm wondering what I'm doing here if I'm offline...