The 0 freq component is just the general, time independent bias of the signal. If it is 0 then of course the integral of something which is neither positively n

The 0 freq component is just the general, time independent bias of the signal. If it is 0 then of course the integral of something which is neither positively nor negatively biased overall will average out to null. The apparent convergence to zero happens because the amount of sample points is exactly the amount of data that can be represented with the number of frequency components you are calculating in your FT. Obviously net error in electronic readings don't really go to zero when you have exactly 1 million of them. Use fewer frequencies in your fft, see the convergence sooner. Use more, see it later. The libraries aren't that impressive. The FFT is simple enough to write in any coding language with basic trig and arithmetic. It could certainly fit into a similar amount of code to what the author has written.