I don’t see the error in your code, but the code here I wrote (plotting half of the fft) works:
fidi = fopen('John Smith emg.txt', 'rt');
T_EMG = textscan(fidi, '%f%f', 'Delimiter',' ');
t = T_EMG{1};
EMG = T_EMG{2};
L = length(t);
Ts = mean(diff(t)); % Sampling Time
Fs = 1/Ts; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
FEMG = fft(EMG)*2/L; % Fourier Transform (Normalised)
Fv = linspace(0, 1, fix(L/2)+1)*Fn; % Frequency Vector
Iv = 1:length(Fv); % Index Vector
figure(1)
plot(Fv, abs(FEMG(Iv)))
grid
