How to plot period on a x axis of a fourier transformed dataset?

Hi,

I have performed a Fourier transform on tide gauge data and I've successfully run the code and produced a plot of frequency (cycles per day) and amplitude (m). In the plot (attached), the largest amplitudes occur at ~2 cycles per day (12 hr and 12.42 hr periods). I would like to plot the period on the x axis and the amplitude on the y axis. How might I accomplish this?

Thanks so much!

Anna

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dt = 1;

sec_per_day = 24*60*60;

Fs = 1/3600;

t = 0:dt:length(data);

fft_tides = fft(Height);

n = length(fft_tides);

P = abs(fft_tides);

P = P(1:n/2+1);

P = P.^2/(Fs*n);

freq = 0:Fs/length(data):Fs/2;

freq = freq*3600*24; % cycles per day, for plotting purposes

%% gives amplitudes in m P2 = abs(fft_tides)/n;

P2 = 2*P2(1:n/2+1);

figure(1) plot(freq,P2)

xlabel('Frequency, (cycles/day)')

ylabel('Amplitude (m)')

title('FFT Sample example')

Answer 1

[Star Strider]

I am not certain what you want.

The period is 1/F, where ‘F’ is frequency, so one way to accomplish that would be to add after the plot:

xt = get(gca, 'XTick');
xp = 1./xt;
set(gca, 'XTick',xt, 'XTickLabel',xp)

If you want increasing values of the period on the x-axis (rather than decreasing as this would plot), use this instead:

set(gca, 'XTick',xt, 'XTickLabel',xp*100, 'XDir','reverse')