Comment 3 for bug 1676902

tbh, it's not obvious. i think this needs to be documented

based on code, the result is the last contiguous sequence where all series overlap. ie if

series1: __, __, __, __, 10, 20, __, 30, 40
series2: 10, 20, 30, 40, 50, 60, 70, 80, 90,

i believe the return value is only the last points. (do not quote me)