title: Putting 2 scale for x-axis in the same plot
body: <p>I've seen examples of how to add 2 scales for y-axis in a single plot, with the command <code>twinx()</code>. However, I still couldn't figure out how to add 2 scales for the x-axis. In my case, I import the [x,y] data from MATLAB and plot them. I would like to have my original data of x(we can call it <code>x1</code>) displayed at the bottom of the figure, and the normalized x-data(we can call it <code>x2</code>) on the top of the figure. My code is:</p>
  
  <pre><code>from scipy.io import loadmat
  import matplotlib.pyplot as plt
  import numpy as np
  
  # loading data
  fid_1 = 'beta_sweep_020.mat'
  m1 = loadmat(fid_1)
  
  x = m1['vt_dX']
  x1 = np.reshape(x, x.size)
  x2 = x1 / (-466.0)
  y = m1['vt_beta']
  y1 = np.reshape(y, y.size)
  
  # plot
  fig = plt.figure()
  ax = fig.add_subplot(111)
  
  plt.plot(abs(x1),y1,'r')
  plt.show()
  </code></pre>
  
  <p>Can anyone help me with this? Thanks.</p>