FFT: Detrend not removing 0 Hz component?

I have a data vector that I am trying to use fft on. I've read in other answers that the 0 Hz component comes from the mean so I need to detrend the data. The following lines should work:

detrend(data,0) or detrend(data,'constant')

but there is no visible change in my output: the 0 Hz component still dominates significantly. If it helps, the vector contains electrical grid frequency values over time. Here is my code:

% FFT Variable Initializations
Fs = 10;    % 10 Hz sampling frequency
[nrows,ncols]=size(data); % Data size
NFFT = 2^nextpow2(nrows); % Next power of 2 from nrows of data

% Perform FFT Analysis on each column
linespec = {'r' 'g' 'b' 'c' 'm' 'y' 'k' '-k'};
for i =1:length(linespec)
    % Detrend the data
    dtr_data = detrend(data(~isnan(data(:,i)),i),'constant');

      % Perform FFT on detrended data
      fft_data = fft(dtr_data,NFFT)/nrows;
      f = Fs/2*linspace(0,1,NFFT/2+1);
      plot(f,2*abs(fft_data(1:NFFT/2+1)),linespec{i})
      hold on    
  end

Thanks in advance for all the help.

[Wayne King]

Can you attach the data, or is it too big? How about just one column.