Gravity: Number of distinct affiliates who earned a commission by referring a paying customer to the publisher's products. This is a weighted sum and not an actual total. For each affiliate paid in the last 8 weeks we add an amount between 0.1 and 1.0 to the total. The more recent the last referral, the higher the value added.
过去8周内每个affiliate卖出一个产品，Gravity就增加0.1-1.0，近期越多的affiliates卖得越多，Gravity增加值越大
越高说明affiliates卖得越多，其他可参考
http://www.ccbbs.com/viewthread.php?tid=9883&highlight=grav
