Actually, on second thought, no points ever seem to be near 0. Well, maybe what I just described isn't EXACTLY how it works, but you should get the idea. If there are 3 months in the year, this is how data plots (y-axis) are calculated.
Code:
$time = 5, 20, 10
$temp = 20, 60, 70
max($time) = 20
max($temp) = 70
$time = 5/20, 20/20, 10/20
$temp = 20/70, 60/70, 70/70
$temp_time = (5/20+20/70)/2, (20/20+60/70)/2, (10/20+70/70)/2