title: matplotlib axis labels not aligned with data bins
body: <p>I am trying to plot a correlation matrix and instead of the bin number as a label (see the y-axis labels) I'd like to have the physical, actual units (the x-axis labels). Unfortunately I can't get to align the xticks with the center of the bins and I don't understand why I don't have a label at each column of the matrix. </p>
  
  <p><img src="https://i.stack.imgur.com/zuV3x.jpg" alt="&quot;5.01&quot; is the last bin label, not the second to last"></p>
  
  <p>The code below produces the figure:</p>
  
  <pre><code>fig = plt.figure()
  ax = fig.add_subplot(1,1,1)
  labels =['', '0.13','0.20','0.32','0.50','0.79','1.26','2.00','3.16','5.01']
  plt.xticks(range(len(labels)+1), labels, size='small')
  #The '+1 in range seems needed as if I don't include it the first label is not displayed at all
  plt.imshow(np.corrcoef(bootstrap_samples_matrix.transpose()), interpolation='nearest', cmap=plt.cm.Reds, extent=(0.5,10.5,0.5,10.5), align="center")
  plt.colorbar()
  </code></pre>