Adalm Pluto: imaging of a frequency range

Hello everyone,

I have an Adalm pluto, with which I want to plot the frequency domain of a specific frequency range (kind of like SDR#)

My goal is to look at that  frequency range and determine the maximum of the function. 

My problem is, that i dont know how to get the data. I already tried with the "iio_readdev" function but i am not able to use the output correctly.

The more I want to explore this subject, the more confused i get. I am not even sure what exactly the output is. 

i am thankfull for any input