flot.threshold: insert generated series after original, not in the end

Thus, prevent it from becoming topmost (it may cause strange looks if
original series is not topmost: some part of it becomes topmost and
overwrites some lines and points that should be above it instead).
Merge pull request #934 from NIA/threshold-fixes

flot.threshold: insert generated series after original, not in the end
Updated credits for #934 threshold overlapping.