1D plot- Postproccessing

Hello,

I am working on a 2D transient problem. I have the results and want to post processing them.
I want to plot my results like this:

I need to detect the minimum of a derived variable all over the domain at a specific time.
then, again detect that variable in another time (and repeat it for my time period)
I wanna plot the derivation of that variable with time and plot it with point graph.

As far as I know I need to have a 1D plot. My problem is that how to find the minimum of the magnitude?
Can I set a function for that or no? if yes, which part?
As a summary, my plot should have the variation of the derived variable on axes(Time VS derived varible)

Thanks
Hooman