title: Create bivariate histogram from binned pandas dataframe
body: <p>I have a pandas dataframe with two columns <strong>a</strong> and <strong>b</strong>. </p>
  
  <pre><code>a    b
  
  23   1.1
  76   2.1
  98   3.4
  18   1.4
  </code></pre>
  
  <p>I have binned b in 10 bins and want to create a histogram consisting of number of <strong>a</strong> values in bins of <strong>b</strong>. </p>
  
  <pre><code>dg = pd.cut(x=df['b'] , bins=10)
  </code></pre>
  
  <p>I want a histogram with x axis as the bins (of <strong>b</strong>) and y-axis as the total <strong>a</strong> values (like 23+18+...) in those bins.
  How should I approach this problem?</p>