Calculate area under FFT graph in MATLAB

Question

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

Answer 1

The FFT is discrete, not continuous - you just need to sum all the bin values. If you're looking at the power spectrum (magnitude squared) then the bin values are in W/Hz, so you would need to multiply each value (or alternatively just the sum), by the bin width in Hz to get power (and hence the total energy in your input sample).