XP experiment

I was trying to figure out how XP gain was calculated. I played a couple of rounds and made this graph. The red points are the ones where I got 0 kills. Some of the times I died and some I quit through the pause menu. It seems that XP is directly correlated with time alive, but maybe kills or leader board time are also involved.
https://drive.google.com/file/d/0B0ETea13k-9yT2NOWjVwYjN2Zkk/view?usp=sharing