title: Matplotlib histogram bins selection depends on whether data is plotted "alone" or with some other data: how so?
body: <p>The title has it. :)</p>
  
  <p>Example:</p>
  
  <pre><code>from numpy import random
  from matplotlib import pyplot as plt
  
  data = [1 + random.randn(1000), random.randn(1000)]
  bins = 10
  
  plt.hist(data, bins, label=['first', 'second'])
  plt.hist(data[1], bins, histtype='step', label=['second again'])
  
  plt.legend()
  
  plt.show()
  </code></pre>
  
  <p>gives ('step' type chosen to aid "viewability", its the same with defaults):</p>
  
  <p><img src="https://i.stack.imgur.com/hP1mK.png" alt="the damn thing ;)"></p>
  
  <p>See?</p>