gnu - FFT output to PSD -

i need understanding output of fft.

i'm working gnu radio. have signal source output float type, after go through fft transform, data send app using udp.

my app receives data in byte array , converts float array. got fft output complex format:

floatarr = {real,img,real,img .....}.

then calculate psd value fft output, , render chart.

but problem chart incorrect, totally different gnu display

here gnu diagram

enter image description here

i calculate psd value formula;

1: magnitude = 2*sqrt(re^2 + img^2)/n

2: psdx = 10*log10(magnitude^2)

but chart different gnu:

enter image description here

please help, have little knowledge of fft/psd (and english too). i've tried render chart weeks, blindly apply formulas.

Tomorrow