How to exactly determine the current value in the FFT-Plot sink

Hi All,

i want to determine the value of the shown signal in my FFT-Plot
sink as exactly as i can. The reading error is to big. If i could zoom
into the FFT-Plot sink it would be better, but i don’t know how to
zoom in. It would be nicer to read the value of the middle of the
FFT-Plot by any python function.

Any Hints ?

Regards Markus