title: Combining two series in pandas along their index
body: <p>I have two series in pandas.</p>
  
  <p>series 1: </p>
  
  <pre><code>id        count_1
  1            3
  3           19
  4           15
  5            5
  6            2
  </code></pre>
  
  <p>and series 2:</p>
  
  <pre><code>id        count_2
  1           3
  3           1
  4           1
  5           2
  6           1
  </code></pre>
  
  <p>How do I combine the tables along the ids to form the below?</p>
  
  <pre><code>id        count_1    count_2
  1            3        3
  3           19        1
  4           15        1
  5            5        2
  6            2        1
  </code></pre>