I threw together a scheme procedure which dumps FFT data to Gnuplot.
You see, however, that I have more work to do as properly labeling the graph, as well as implementing averaging for a more practical spectrum display. The 512 points on the x-axis correspond to the 512 complex numbers in the FFT output buffer, but center frequency in this case is actually 103Mhz, at 8 msps.
scheme@(guile-user)> (define d (hackrf-open))
scheme@(guile-user)> (hackrf-sensible-defaults d)
scheme@(guile-user)> (hackrf-set-freq d 103000000)
scheme@(guile-user)> (hackrf-set-baseband-filter-bandwidth d 2000000)
scheme@(guile-user)> (hackrf-enable-amp d)
scheme@(guile-user)> (define c (cb-fft-to-gnuplot))
scheme@(guile-user)> (hackrf-start-rx d c)
scheme@(guile-user)> (hackrf-stop-rx d)It was mostly easy to write the scheme -> Gnuplot interface as that is just sending text commands to Gnuplot standard input. However, it took me a while to fix an elusive bug in my C FFT function.
The Libre Hacker 