title: Seaborn - Modify y axes by a factor in a histogram
body: <p>I am trying to present a histogram plot using <code>FacetGrid</code> My question is how can I divide the y-axes by a factor. For example in the data below: let's said that 
  I need to normalize the y-axes, dividing site <code>a</code> by 2 and site <code>b</code> by 5.
  How can I modify the y-axes and get the new values of all plots? I'll appreciate any insights</p>
  
  <pre><code>dfpp = pd.DataFrame({'site': ['a', 'b', 'a', 'a', 'b', 'b', 'a', 'b', 'b'],
                    'lenght': [3, 4, 5, 7,4, 9,4,3,12]})
  
  g = sns.FacetGrid(dfpp, col="site")
  g = (g.map(plt.hist, "lenght",bins=3))
  </code></pre>