How to Plot fft of ascii values in MATLAB? [closed]

I have a ascii file containing 2048 x and y value pairs. I just want to know how to plot fft of y in MATLAB. I am writing following MATLAB code but could not be able to find appropriate result.

How can I do this? This is what I have tried:

I = load('data1.asc');

for i = 1:2048
    y = I(:,2);
end

plot(x)

Fs = 40000;                    
T = 1/Fs;                   
L = 2000;     
NFFT = 2^nextpow2(L);
Y = abs(fft(y,NFFT))/L;
f = Fs/2*linspace(0,1,NFFT/2+1);

figure, plot(f,2*abs(Y(1:NFFT/2+1))) 
axis([0 40000 0 40])
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')

Instead of diving straight into MATLAB's FFT routine you should instead consider using the periodogram function. When people say "FFT" they usual mean PSD or periodogram, i.e. a plot of power spectral density using a suitably windowed and FFTed sample. The periodogram function in MATLAB takes care of all the details of this for you, i.e. applying a window function, calculating the FFT, deriving magnitude from FFT output, appropriate scaling of axes, and even plotting if required.

Note: periodogram is in the MATLAB Signal Processing Toolbox - if you do not have access to this then you can either consider using Octave (free MATLAB clone) which has a periodogram clone, otherwise you would need to put the various building blocks together yourself:

window function
FFT
calculate magnitude
take scare of scaling of freq and magnitude values
plot PSD

The fft part of the code looks good to me. However, this bit doesn't make much sense:

for i = 1:2048
    y = I(:,2);
end

What are you trying to do here? You're not using the loop index i at all in the for loop.

Also, I assume y is of length 2000, can you confirm? Otherwise L = 2000 should be changed to L = length(y). Similarly, I assume that the sampling frequency of the data is 40kHz, otherwise Fs = 40000 is not correct.

EDIT following discussion in comments:

With the data that you have provided, I get the same results. The only thing I did is exclude the last data point from the analysis when it drops to zero. The way you read the data still doesn't make sense to me. Note I am using Octave, not MATLAB, but the code should give the same results in MATLAB.

load('ascii_value.txt')
y = ascii_value(1:end-1,2);
plot(y)
L=length(y);
Fs = 40000;
T = 1/Fs;
NFFT = 2^nextpow2(L);
Y = abs(fft(y,NFFT))/L;
f = Fs/2*linspace(0,1,NFFT/2+1);
figure, plot(f,2*abs(Y(1:NFFT/2+1)))
axis([0 40000 0 40])
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')

The signal looks like this: