[Discuss-gnuradio] how to calculate the peak values of a signal spectrum by a grc project

usrp_fft.py is one the examples (or utilities) which is prepared in gnuradio source code.
this example python utility is in this path:
<gnuradio source directory>/gr-utils/src/python/usrp_fft.py

this python example calculates the fft transform of the signal which is captured from the USRP (as source); and then, plots the spectrum (output of fft transform) via a GUI.
one the options prepared on the GUI is "Peak Hold"; that can be selected by checking its checkbox.
by checking the Peak Hold checkbox, a green diagram will be ploted on the GUI, that indicates the maximum values of the spectrum in every frequency.

I made a project in grc that generates a python code like usrp_fft.py.
but now i need to catch the Peak Hold Diagram values as a vector; to do sum calculations on these values.
i tried to use some blocks prepared by grc, like "Operator -> FFT" or "Operator -> Log Power FFT", to calculate the fft spectrum of the USRP signal; but i dont know how to manipulate the output of these blocks and how to catch the Peak Values?

your help will be appreciated
thnx