title: Add missing date index in dataframe
body: <p>I have loaded a CSV files with index datetime which is the last day of months in a year. I wanted to fill missing dates with empty values as rows.</p>
  
  <p>Following is my CSV file structure</p>
  
  <pre><code>Date    Australia   China
  2011-01-31  4.75    5.81
  2011-02-28  4.75    5.81
  2011-03-31  4.75    6.06
  2011-04-30  4.75    6.06
  </code></pre>
  
  <p>I want to fill all the dates in a month with empty columns.</p>
  
  <p>I tried the following code but it is not working.</p>
  
  <pre><code>import pandas as pd
  df = pd.read_csv("data.csv", index_col="Date")
  df.reindex(pd.date_range("2011-01-01", "2011-10-31"), fill_value="NaN")
  </code></pre>