title: merging 2 dataframes vertically
body: <p>I have 2 dataframes that have 2 columns each (same column names). I want to merge them vertically to end up having a new dataframe.</p>
  
  <p>When doing  </p>
  
  <pre><code>newdf = df.merge(df1,how='left',on=['Col1','Col2'])
  </code></pre>
  
  <p>The new df has only the rows from <code>df</code> and none of the rows from <code>df1</code>. Any reasons why this might happen?</p>
  
  <pre><code>Col1    Col2
  asd     1232
  cac     2324
  .....
  </code></pre>
  
  <p>and the <code>df1</code> is:</p>
  
  <pre><code>Col1    Col2
  afaf    1213
  asas    4353
  </code></pre>
  
  <p>The new dataframe <code>newdf</code> should be:</p>
  
  <pre><code>Col1   Col2
  asd     1232
  cac     2324
  afaf    1213
  asas    4353
  </code></pre>