title: Vertically merge dataframes
body: <p>I have a query regarding merging 4 dataframes 
  For example i have 4 dataframes as below :</p>
  
  <pre><code>print(df1)
  
  SET I      Violations
  Rule 1            1
  Rule 2            1
  Rule 3            6
  
  print(df2)
  
  SET II       Violations
  Rule 1            2
  Rule 2            3
  Rule 3            6
  
  print(df3)
  
  SET III       Violations
  Rule 1            2
  Rule 2            4
  Rule 3            8
  
  
  print(df4)
  
  SET IV       Violations
  Rule 1            2
  Rule 2            5
  Rule 3            8
  </code></pre>
  
  <p>My expected output :</p>
  
  <pre><code>SET I      Violations
  Rule 1            1
  Rule 2            1
  Rule 3            6
  
  
  SET II       Violations
  Rule 1            2
  Rule 2            3
  Rule 3            6
  
  SET III       Violations
  Rule 1            2
  Rule 2            4
  Rule 3            8
  
  SET IV       Violations
  Rule 1            2
  Rule 2            5
  Rule 3            8
  </code></pre>
  
  <p>Outputs i am getting right now :</p>
  
  <p>a)</p>
  
  <pre><code>SET I   SET II    SET III   SET IV  Violations
  Rule 1                                     1
  Rule 2                                     1
  Rule 3                                     6 
          Rule 1                             2
          Rule 2                             3
          Rule 3                             6
           Rule 1                    2
                   Rule 2                    4
                   Rule 3                    8
                         Rule 1          2
                             Rule 2          5
                             Rule 3          8
  </code></pre>
  
  <p>command_used:</p>
  
  <pre><code>pandas.concat([df1,df2,df3,df4],axis=0,ignore_index=True)
  </code></pre>
  
  <p>b)  </p>
  
  <pre><code>Rule 1  1   Rule 1    2    Rule 1  2      Rule 1  2 
  Rule 2  1   Rule 2    3    Rule 2  4      Rule 2  5
  Rule 3  6   Rule 3    6    Rule 3  8      Rule 3  8
  </code></pre>
  
  <p>command_used:</p>
  
  <pre><code>pandas.concat([df1,df2,df3,df4],axis=1,ignore_index=True)
  </code></pre>
  
  <p>Please help me with above</p>