title: Cumulative list by dates
body: <p>I have a list of products by date - </p>
  
  <pre><code>2020-01-01   A
  2020-01-02   B
  2020-01-02   C
  2020-01-02   D
  2020-01-03   B
  2020-01-03   C
  2020-01-03   E
  </code></pre>
  
  <p>I want the results to look like - </p>
  
  <pre><code>2020-01-01   A
  
  2020-01-02   A
  2020-01-02   B
  2020-01-02   C
  2020-01-02   D
  
  2020-01-03   A
  2020-01-03   B
  2020-01-03   C
  2020-01-03   D
  2020-01-03   E
  </code></pre>
  
  <p>cumulative list of all products by date.
  What would be the most efficient way to solve this</p>