title: Aggregating two Pandas series by summing
body: <p>I am trying to sum two series that have some matching indexes, but some that are unique.  e.g.:</p>
  
  <pre><code>a = pd.Series([0.2, 0.1, 0.3], index=['A', 'B', 'C'])
  b = pd.Series([0.2, 0.2], index=['A', 'D'])
  </code></pre>
  
  <p>Notice that index A is in both a, and b.  I want to end up with a new series, which has the summed up aggregate of all indices:</p>
  
  <pre><code>A    0.4
  B    0.1
  C    0.3
  D    0.2
  dtype: float64
  </code></pre>
  
  <p>notice index A is the sum of both a and b (0.2 + 0.2), whereas B, C, and D are the original value.  If I try to do:</p>
  
  <pre><code>c = a + b
  </code></pre>
  
  <p>I get the proper value for index A, but NaN for all other values.  Any thoughts on the best way to do this?</p>