title: Fourier transform of a time sequence data shows a diagonal line in frequency space
body: <p>I have sets of data with 1000 equally spaced points (in time-space) and was able to get its Fourier transform (in frequency-space), but the problem is that one set of data shows a diagonal line which passes from the last point of the right hand side back to the last point of the left hand side. I tried lowering the samples by only taking the last 500 points, but it seems to be around even after taking only the last 100 points. Thus maybe it's not sample dependent, but rather something's lacking/wrong with my syntax.</p>
  
  <p><a href="https://i.stack.imgur.com/HV9YC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HV9YC.png" alt="enter image description here"></a></p>
  
  <p>FFT was called by the 3 lines below (which I got from other posts)</p>
  
  <pre><code>sp  = np.fft.fft(y1_500)
  freq= np.fft.fftfreq(y1_500.shape[-1])
  plt.plot(freq, np.abs(sp))
  </code></pre>
  
  <p>Can anyone tell me what's with the diagonal line?</p>