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).

This picture describes the issue the best:
![screen8](https://f.cloud.github.com/assets/26939/78091/70c5ec56-616e-11e2-870a-bbdc81a802fc.png)

It is generated in this jsFiddle: http://jsfiddle.net/nia_ru/z5YC6/