Real time spectrum plot

Hi all,

I want to make a fft realtime plot with usrp but, i want to do fft job
myself (for trying different ffts) and plot the spectrum.
I tried to use qtgui_sink_c but it takes fft itself. And when i tried
the time domain plot after take the fft myself the graph slides (i think
because it show sample by sample).

I want to show for example 256 sample than the following 256 samples
goes on like this.