title: Pandas dataframe merging but creating NaN values
body: <p>I have two pandas dataframes with one column in each which looks like:</p>
  <pre class="lang-py prettyprint-override"><code>df1                        df2
  Price                     volume
   20                        200
   12                        50
   43                        10
   45                        5
  </code></pre>
  <p>When I try to merge the two dataframes using: new_df = pd.concat([Volume, Price], axis=1) i get</p>
  <pre class="lang-py prettyprint-override"><code>    Volume      Price
  3   20           NaN
  4   NaN          200
  5   12           NaN
  6   NaN          50
  7   43           NaN
  </code></pre>
  <p>I've tried using pd.concat / pd.merege / pd.join but can't seem to get a clean merge. whats the best way to go about fixing this?</p>