title: Plotting with matplotlib specgram?
body: <p>I'm trying to plot a signal and the spectrogram of the signal with matplotlib, but... i get the spectrogram only for the first value (samples) of my signal (like 60 of the 30000...).</p>
  
  <p>It's a very long file, that's why I would like to plot only the first 30000 sample.</p>
  
  <p>here is the code :</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  import pandas as pd
  
  Data=pd.read_csv('MySignal.txt', 
      skiprows=[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19], 
      header=0)
  print(Data.head())
  DataI=Data['Sig'].tolist()
  print(len(Data.index))
  DataI=DataI[0:30000]
  
  NFFT = 200     # the length of the windowing segments
  Fs = 500  # the sampling rate
  
  # plot signal and spectrogram
  
  t=range(len(DataI))
  ax1 = plt.subplot(211)
  plt.plot(t, DataI)
  plt.subplot(212, sharex=ax1)
  Pxx, freqs, bins, im = plt.specgram(DataI, NFFT=NFFT, 
                          Fs=Fs,noverlap=100, cmap=plt.cm.gist_heat)
  plt.show() 
  </code></pre>
  
  <p>I don't understand well how plt.specgram work, so i don't understand where is the problem... </p>
  
  <p>Thanks a lot !</p>