Graphical interpretation of magnitude (derived from fft)

Hello to all users of Matlab.

I have 20 years of temperature measurements. Twice a day (max and min) with 12 hours between measurements. To understand which frequencies are more pronounced, I applied the following code:

x = detrend(x);                    
n = length(x);               
nfft = 2^nextpow2(n);
Fs = 1/(12*60*60);             
f = 0 : Fs/(nfft-1) : Fs/2;
Y = fft(x, nfft);           
Y = Y(1:((nfft+1)/2));
Y = Y/(nfft/2);
Ya = abs(Y);            

figure(100)
plot(f, Ya); xlabel('Frequency (Hz)'); ylabel('|Y(f)|'); grid on

I get:

The energy is concentrated in the last frequency (Nyquist, if I'm not mistaken). Is there any error in the code? Not the first time I do this, but never for this type of data and with a sample rate (Fs) so small.

Thanks for the help