title: How you make a circle in the middle of the graph
body: <p>I am plotting the contour line, if I want to make a circular area in the middle to represent the water output what should I do?</p>
  <p>My for loop function does not give me what I want.</p>
  <p><strong>This is my code:</strong></p>
  <pre><code>import numpy as np 
  import matplotlib.pyplot as plt
  x=np.linspace(-100,100,200)
  y=np.linspace(-100,100,200)
  X,Y= np.meshgrid(x,y)
  for Z in X&lt;-40.0,X&gt;40.0:
     for Z in Y&lt;-40.0,Y&gt;40.0:
            Z=np.sqrt(X**2+Y**2)
            #Z = 1*np.sqrt(X**2+Y**2)*(1-((40*40)//(X**2+Y**2)))*np.sin(np.arctan(Y//X))
      cp=plt.contourf(X,Y,Z)
      plt.colorbar(cp)
      plt.show
  </code></pre>