title: Add a array-based figure over an image with pyplot
body: <p>I am plotting a DICOM slice from a CT on which I want to draw a Region of Interest (ROI). The problem is that I didn't succeed into displaying this ROI over my background DICOM image yet. The idea is to finally draw a colored circle on my background DICOM image. 
  Here is the code:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  ArrayDicom = np.ones((512, 512, 148))
  plt.figure(figsize=(3, 3), dpi=300)
  plt.axes().set_aspect('equal', 'datalim')
  plt.axis('off')
  plt.set_cmap(plt.gray())
  x = np.arange(0, 512, 1)
  y = np.arange(0, 512, 1)
  plt.pcolormesh(x, y, np.flipud(ArrayDicom[:, :, 0]))  # ArrayDicom = Image 3D Array
  
  nx, ny, nz = (512, 512, 148)
  # Grid creation
  x, y = np.ogrid[:nx, :ny]
  # Mask creation
  radius = 5
  x_center, y_center = (2, 2)
  contour = (x - x_center) ** 2 + (y - y_center) ** 2 == radius ** 2
  mask_contour = np.zeros((nx, ny, nz))
  mask_contour[:, :, 0][contour] = 1
  ax = plt.gca()
  ax.plot(mask_contour[:, :, 0], color='red')
  
  plt.show()
  </code></pre>
  
  <p>I used this tutorial <a href="https://pyscience.wordpress.com/2014/09/08/dicom-in-python-importing-medical-image-data-into-numpy-with-pydicom-and-vtk/" rel="nofollow noreferrer">https://pyscience.wordpress.com/2014/09/08/dicom-in-python-importing-medical-image-data-into-numpy-with-pydicom-and-vtk/</a> to display my DICOM images.
  I think I just miss a way of plotting my datas. I know I can use <code>plt.Circle</code> but I would like to deal with sphere instead of circle next. </p>
  
  <p>Thanks for help.</p>