title: How to add lines to contour plot in python `matplotlib`?
body: <p>I have the following function to illustrate some contour lines :</p>
  
  <pre><code>"""
  Illustrate simple contour plotting, contours on an image with
  a colorbar for the contours, and labelled contours.
  
  See also contour_image.py.
  """
  import matplotlib
  import numpy as np
  import matplotlib.cm as cm
  import matplotlib.mlab as mlab
  import matplotlib.pyplot as plt
  
  matplotlib.rcParams['xtick.direction'] = 'out'
  matplotlib.rcParams['ytick.direction'] = 'out'
  
  X = np.arange(-1.2, 1.2, 0.005)
  Y = np.arange(-1.2, 1.2, 0.005)
  X, Y = np.meshgrid(X, Y)
  Z = (np.ones([np.shape(X)[0],np.shape(X)[1]])-X)**2+100*(Y-(X)**2)**2
  
  
  # Create a simple contour plot with labels using default colors.  The
  # inline argument to clabel will control whether the labels are draw
  # over the line segments of the contour, removing the lines beneath
  # the label
  levels = np.arange(-100.0, 600, 1.0)
  plt.figure()
  CS = plt.contour(X, 
                   Y, 
                   Z,
                   levels=levels,
                  )
  plt.clabel(CS, 
             np.array(filter(lambda lev: lev &lt;5.0, levels)),
             inline=0.5, 
             fontsize=10,
             fmt='%1.1f'
            )
  
  plt.hold(True)
  
  
  plt.plot(np.arange(-1.0, 1.0, 0.005),
          np.arange(-1.0, 1.0, 0.005),
          np.ones(len(np.arange(-1.0, 1.0, 0.005)))*100, '-k')
  
  plt.title('Contour Lines and Constraint of Rosenbrock Optimiztion Problem')
  plt.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/pomiv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/pomiv.png" alt="enter image description here"></a></p>
  
  <p>The contour plot looks great if you comment out the lines....:</p>
  
  <pre><code># plt.hold(True)
  
  
  # plt.plot(np.arange(-1.0, 1.0, 0.005),
  #         np.arange(-1.0, 1.0, 0.005),
  #         np.ones(len(np.arange(-1.0, 1.0, 0.005)))*100, '-k')
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/asziR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/asziR.png" alt="enter image description here"></a></p>
  
  <p>...but I cannot get the lines to show up overlayed on the plot like I need them.  I just simply need them to be overlayed on top of the contour plot. What is the best way to do this?  </p>
  
  <p>I know it is <a href="https://stackoverflow.com/questions/28458192/r-add-a-line-to-contour-plot">possible in R</a>, but how to do this in <code>Python</code> using <code>matplotlib</code>?</p>