title: How to add third level of ticks in python matplotlib
body: <p>Matplotlib axes have Major and Minor ticks. How do I add a third level of tick below Minor?</p>
  
  <p>For example</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  import matplotlib.ticker
  
  t = np.arange(0.0, 100.0, 0.1)
  s = np.sin(0.1*np.pi*t)*np.exp(-t*0.01)
  
  fig, ax = plt.subplots()
  plt.plot(t, s)
  
  ax1 = ax.twiny()
  ax1.plot(t, s)
  
  ax1.xaxis.set_ticks_position('bottom')
  
  majors = np.linspace(0, 100, 6)
  minors = np.linspace(0, 100, 11)
  thirds = np.linspace(0, 100, 101)
  
  ax.xaxis.set_major_locator(matplotlib.ticker.FixedLocator(majors))
  ax.xaxis.set_minor_locator(matplotlib.ticker.FixedLocator(minors))
  ax1.xaxis.set_major_locator(matplotlib.ticker.FixedLocator([]))
  ax1.xaxis.set_minor_locator(matplotlib.ticker.FixedLocator(thirds))
  ax1.tick_params(which='minor', length=2)
  ax.tick_params(which='minor', length=4)
  ax.tick_params(which='major', length=6)
  ax.grid(which='both',axis='x',linestyle='--')
  
  plt.axhline(color='gray')
  
  plt.show()
  </code></pre>
  
  <p>produces the effect I want using twinned x-axes. </p>
  
  <p><a href="https://i.stack.imgur.com/yoblW.png" rel="noreferrer"><img src="https://i.stack.imgur.com/yoblW.png" alt="Third level ticks"></a></p>
  
  <p>Is there a better way? </p>