Extremely spiky graph in stats tab..is this normal

Most of the time this graph looks more or less flat with some surge of activity here and there but if i stand still for too long, after a while the graph becomes extremely spiky with or without any shaders enabled. Is there any circumstance where this would make sense or is something just wrong with the game engine or something?

P.s - What does the y-axis of this graph actually represent, i know the x-axis is time but not sure about y-axis.

I would also be interested for some info about that graph.

As far as I know this graph shows the calculated framerate over the last two seconds.

Y is FPS. So yes, it shows the average framerate over time. It being spiky means you have an unstable framerate (aka the time it takes for a frame to compute flunctuates a lot).