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:
/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