title: Python Merge columns into one column by duplicating date indexes
body: <p>I want to merge columns into one column by adding rows with the same index. Of course, the index is the date. Then I want to plot the <code>seaborn</code> plots.</p>
  <p>My data:</p>
  <pre><code>df = 
                A    B    # A, B two unique groups, denote 36, 24 size
  timestamp                       
  2019-10-01   10   20
  2019-10-02   30   40
  </code></pre>
  <p>Expected answer:</p>
  <pre><code>df = 
              val    group   size    # A, B denote 36, 24 size
  timestamp                       
  2019-10-01   10    A       34
  2019-10-01   20    B       24
  2019-10-02   30    A       34
  2019-10-02   40    B       24
  </code></pre>
  <p>My code:</p>
  <p>I have no idea how to proceed here.</p>