title: combine dataframe in the bottom of another dataframe
body: <p>What I would like to combine both of this below dataframe.</p>
  
  <pre><code>    &gt; am_R3_1
                   Group.2  x.x      x.y
       2   clearterminate     3   21
       3  display.cryptic    86   30.127
       4            price    71   898
      41               AM   160   316.37
  
      &gt; am_R3_2
        Group.2  x.x  x.y
      1    ping    1    5
      2      1Y    1    5
  </code></pre>
  
  <p>I would like to get the final result smiliar to below for all Group.1 . Please advise which function I can use to get the following result ?</p>
  
  <pre><code>        Group.2          x.x   x.y
       2  clearterminate     3    21
       3  display.cryptic   86    30.127
       4  price             71   898
      41  AM               160   316.375
      1   ping               1     5
      2   1Y                 1     5
  </code></pre>