Unexplained Line On Graph

[PeteCress]

I see three documented lines on my graph plus an off-green annotation for "Process Restraint".   Red="Processor Use", Green="Repsonsiveness", and Blue="Memory Load".   All of these are documented in the Right-Click popup that appears in the upper left of the graph.

But sometimes, at the bottom of the graph, I see a grey or off-white line.   I don't see it right now or I'd post a screen snap.

Can anybody say what that line represents?

[hanemach_gt]

This question has already been answered here.