Hi,
Here is a correct and complete code for fft calc and plot with your data given in .xls data file:
clearvars; clf;
D=readtable('Messdaten.xls');
time=D.Var1; % Time
X = D.Var2; % Data
Fs=1e3; % Sampling frequency, [Hz]
L = length(X);
N = 2^nextpow2(L);
Y = fft(X, N);
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 S(t)')
xlabel('f (Hz)')
ylabel('|P1(f)|')
