title: Calculate area under FFT graph in MATLAB
body: <p>Currently I did a FFT of a set of data which gives me a plot with frequency at x axis and amplitude at y axis. I would like to calculate the area under the graph to give me the energy. </p>
  
  <p>I am not sure how to determinate the area because I am without the equation and also I only want a certain area of the plot rather than whole area under the plot. Is there a way I can do it?</p>