function[F,Xplot,h]= fftplot(x,M,deltat,pro)

% FFTPLOT calculates and plots a meaningful frequency domain
% representation of a signal.
%
% usage: FFTPLOT(x,M,deltat,proportion), 
%   x=input signal, M=M point FFT, deltat=sampling interval, equal to 1/fs
%   where fs is the sampling frequency, proportion= ratio of fs to plot to.
%   
% if you want to plot 0 to fs/10, pro is equal to 10..
%
% Example usage, given a signal x and a time vector t and relatively low
% frequencies:
%
%   fft(x,length(x),t(2)-t(1),2);
%
% This assumes that the sampling interval is constant, has NOT been
% modified from the experiment. Equivalently, you could use
% length(t)/t(end). This will plot the spectrum of x from 0Hz to half 
% of the sampling frequency. You can also use M>length(x), but no
% information will be gained...it may just look nicer.

X=fft(x,M); X=deltat*X; % calculate the FFT and scale.
deltaf=deltat^-1/M; % determine the sampling frequency
upperbound=round(M/pro); % how far do we plot?

kplot=0:upperbound+1; % lets plot a little further
Xplot=X(1:upperbound+2); % matlab doesn't start counting at 0.

F=deltaf*kplot; % convert to Hz

% plot the magnitude of the spectrum. if you want to plot the so called 
% power spectral density, you should change to using semilogy as that is 
% customary.

% also useful in analysis is to plot the phase of the signal, which can be
% accompished by plotting angle(Xplot).

h=plot(F,abs(Xplot)); xlabel('Frequency (Hz)'); grid; 
axis([0 deltat^-1/pro 0 max(abs(X))+0.1*max(abs(X))]); %trim.