title: How to format contour lines from Matplotlib
body: <p>I am working on using Matplotlib to produce plots of implicit equations (eg. y^x=x^y). With many thanks to the help I have already received I have got quite far with it. I have used a contour line to produce the plot. My remaining problem is with formatting the contour line eg width, color and especially zorder, where the contour appears behind my gridlines. These work fine when plotting a standard function of course.</p>
  
  <pre><code>import matplotlib.pyplot as plt 
  from matplotlib.ticker import MultipleLocator, FormatStrFormatter
  import numpy as np 
  
  fig = plt.figure(1) 
  ax = fig.add_subplot(111) 
  
  # set up axis 
  ax.spines['left'].set_position('zero') 
  ax.spines['right'].set_color('none') 
  ax.spines['bottom'].set_position('zero') 
  ax.spines['top'].set_color('none') 
  ax.xaxis.set_ticks_position('bottom') 
  ax.yaxis.set_ticks_position('left') 
  
  # setup x and y ranges and precision
  x = np.arange(-0.5,5.5,0.01) 
  y = np.arange(-0.5,5.5,0.01)
  
  # draw a curve 
  line, = ax.plot(x, x**2,zorder=100,linewidth=3,color='red') 
  
  # draw a contour
  X,Y=np.meshgrid(x,y)
  F=X**Y
  G=Y**X
  ax.contour(X,Y,(F-G),[0],zorder=100,linewidth=3,color='green')
  
  #set bounds 
  ax.set_xbound(-1,7)
  ax.set_ybound(-1,7) 
  
  #add gridlines 
  ax.xaxis.set_minor_locator(MultipleLocator(0.2)) 
  ax.yaxis.set_minor_locator(MultipleLocator(0.2)) 
  ax.xaxis.grid(True,'minor',linestyle='-',color='0.8')
  ax.yaxis.grid(True,'minor',linestyle='-',color='0.8') 
  
  plt.show() 
  </code></pre>