title: adding line plot to imshow and changing axis marker
body: <p>I have made the attached plot using the following codes:</p>
  
  <pre><code>a = 1
  theta = np.linspace(0,2*np.pi,101)
  x = np.linspace(-3*a,3*a,1001, dtype='complex')
  y = np.linspace(-3*a,3*a,1001, dtype='complex')
  X,Y = np.meshgrid(x,y)
  
  # come manipulations with V
  # (same shape and type as X,Y) not shown here 
  
  plt.subplot(1,2,1)
  plt.scatter(a*np.cos(theta), a*np.sin(theta))
  plt.imshow(V.real)
  plt.colorbar()
  plt.subplot(1,2,2)
  plt.scatter(a*np.cos(theta), a*np.sin(theta))
  plt.imshow(V.imag)
  plt.colorbar()
  </code></pre>
  
  <p>What I want to do is that: </p>
  
  <p>1) change the scale of the plot such that the horizontal and vertical axis varies between -3*a and 3*a</p>
  
  <p>2) plot the circle boundary (centered at the origin with radius = a). Now it appears at the top left, as the scale of the plot is changed from [-3*a,3*a] to that of the size of the array.<a href="https://i.stack.imgur.com/1JbMX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1JbMX.png" alt="enter image description here"></a> </p>