title: How to plot spectrum using FFTW3/ QWT?
body: <p>I want to plot the frequency spectrum  (like they do for example in Audacity). Hence I want the frequency in Hertz on the x-axis and the amplitude  on the y-axis.
  My input is a periodically sine wave with 0,7 as  amplitude and 500HZ as frequency.I use FFTW to compute the magnitude and QWT  to plot.
  My problem , what parameters should I put in setSamples to get a pic on 500 HZ ? 
  Any help  would be appreciated
  here is my code </p>