title: shade region of interest in matplotlib chart
body: <p>Given a plot like</p>
  
  <pre><code>import matplotlib
  import matplotlib.pyplot as plt
  import numpy as np
  t = np.arange(0.0, 2.0, 0.01)
  s = 1 + np.sin(2 * np.pi * t)
  fig, ax = plt.subplots()
  ax.plot(s)
  ax.set(xlabel='time (s)', ylabel='voltage (mV)', title='sine')
  ax.grid()
  plt.show()
  </code></pre>
  
  <p>How can I shade the vertical slices (from bottom to top) of the chart where the y-value of the plot is between (e.g.) 1.25 and 0.75 automatically?</p>
  
  <p>the sine is just a sample here, the actual values of the plot are less regular.</p>
  
  <p>I have seen <a href="https://stackoverflow.com/questions/23248435/fill-between-two-vertical-lines-in-matplotlib">FIll between two vertical lines in matplotlib</a>, which looks similar to this question, but the answer there shades a region between fixed x values. I want the shaded region to be determined by the y values.</p>