title: log2 axis doesn't work for histograms in matplotlib/seaborn
body: <p>when plotting a histogram using matplotlib/seaborn i would like to change the x-axis to be in log2 scale. the data plotted are <a href="http://pastebin.com/GDLM0tQK" rel="nofollow">here</a></p>
  
  <p>when i take the log2 of values and make a histogram it works, but if i plot the unlogged values and change the x-axis using <code>set_xscale</code>, it gives the wrong result. code is:</p>
  
  <pre><code>import numpy as np
  import seaborn as sns
  import matplotlib.pylab as plt
  
  df = pandas.read_table("data.csv",sep="\t")
  plt.figure()
  sns.set_style("ticks")
  ax1 = plt.subplot(2, 1, 1)
  plt.hist(df["y"])
  ax1.set_xscale("log", basex=2)
  ax2 = plt.subplot(2, 1, 2)
  plt.hist(np.log2(df["y"]))
  </code></pre>
  
  <p>the plot:</p>
  
  <p><a href="https://i.stack.imgur.com/qTlyZ.png" rel="nofollow"><img src="https://i.stack.imgur.com/qTlyZ.png" alt="enter image description here"></a></p>
  
  <p>is this a bug or did i change axes incorrectly?</p>