How to plot a single spectrum using FFT from waveform?

clc;
data = csvread('data.csv') ;
t = data(:,1) ;
amp = data(:,2) ;
L = length(t) ;
N = length(amp);
dt = max(diff(t)) ;
Fs = 1/dt ;
%%1st coding

fs=1/2E-5;
t=0:0.0005:1;
fs=1/(t(2));
x3 = sin (2*pi*60*t);

x=amp;
nfft = 1048576;

X=fft(x,nfft);
X=X(1:nfft/2);

mx = abs (X);
f = (0:nfft/2-1)*fs/nfft;

figure,plot(x);
title ('Sine Wave');
xlabel('Time');
ylabel('Frequency');
figure,plot (f,mx);
title ('Power Spectrum of a Sine Wave');
xlabel('Frequency');
ylabel('Power');

%%2nd Coding
Y = fft(amp);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = Fs*(0:(L/2))/L;
plot(f,P1) 
title('Single-Sided Amplitude Spectrum of X(t)')
xlabel('f (Hz)')
ylabel('|P1(f)|')

is there any problem for these coding? any solution?