How to plot a graph from a text file saved in a for loop?

Hello everyone, I want to plot a graph from a text file which i save in my matlab code. The code goes something like this.

pause on
  for i=1:50

    % calculations %
    .......

    % saving a file %
    name='output.txt';
    fid = fopen(name,'a'); 
    FORMAT = '%g ';
    if i==1;
    fprintf(fid, 'Date(m/d) Time(24hr) Carbon Money Grid Batt\n\n');
    end
    fprintf(fid,FORMAT,c_fc(2:3),c_fc(4),Carbon_saved(i),Money_saved(i), Grid(i), Batt(i));
    fprintf(fid,'\n');

   %%Plotting Graphs

if i<24
  V=output(1:i,2);
  U=output(1:i,5);
else 
  V=output(i-23:i,2);
  U=output(i-23:i,5);
end

    plot(V,U);

fclose(fid);
i=i+1;

pause(3600)

end
pause off

I want to draw a graph such that after every iteration it updates itself for last 24 values of the file. But this code gives error that "undefined variable output". Can someone help me in understanding how I can generate graph for the above condition. Also, I want to generate graph for the column 3 and 4 with respect to column 2. But, I think it will follow the same logic as before.

[Adam]

It's not obvious why a file needs to be involved since you are writing it rather than reading it.

'output' is not defined because it is just the name you assigned to a text file.

If you use a matrix variable, output , instead and remove all use of file it would be a lot simpler.