The Transmogrifox's code is given following:

//Frequency components of a signal

//----------------------------

// build a noides signal sampled at 1000hz containing to pure frequencies

// at 50 and 70 Hz

sample_rate=1000;

t = 0:1/sample_rate:0.6;

N=size(t,'*'); //number of samples

s=sin(2*%pi*50*t)+ 0.3*sin(2*%pi*70*t+%pi/4)+ 0.2*grand(1,N,'nor',0,1);

y=fft(s);

//the fft response is symetric we retain only the first N/2 points

f=sample_rate*(0:(N/2))/N; //associated frequency vector

n=size(f,'*')

yy = y*2/N;

clf()

plot2d(f,abs(yy(1:n)))

And the following picture is the result.

So, if anyone has any question, I think I and Transmogrifox are ready for help.