title: How to plot without extending axis limits?
body: <p>I'm trying to draw on an existing axis without extending or modifying its limits.</p>
  
  <p>For example:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  xy = np.random.randn(100, 2)
  
  plt.scatter(xy[:,0], xy[:,1])
  </code></pre>
  
  <p>Makes a fine plot with well-fitting axis limits.</p>
  
  <p>However, when I try to draw a line on top of it:</p>
  
  <pre><code>xlim = plt.gca().get_xlim()
  plt.plot(xlim, xlim, 'k--')
  </code></pre>
  
  <p>the axis limits are extended, presumably to create padding around the new data.</p>
  
  <p>How can I draw a line without this padding?</p>