title: Filling missing lines with "nan" with pandas reindex
body: <p>Thanks to Jeff, I could find the missing lines in my file data structure and fill the missing lines.</p>
  
  <p>However, filled missing lines in output show "2013-07-01 00:00:00,,,,,,,,,,,,,,,,,,", not with nan. I would like to fill "NaN" or "nan" inside of commas.</p>
  
  <p>My code is:</p>
  
  <pre><code>filin = DataPath + 'SKP_AWS_MIN_QC_10001_2013.07-09.DAT'
  pd.set_option('max_rows',10)
  data=pd.read_csv(filin,sep='#',index_col=[1],parse_dates=[1])
  print data
  index = pd.date_range('2013-07-01 00:00:00','2013-09-30 23:59:00',freq="T")
  df = data
  sk_f = df.reindex(index)
  print sk_f
  sk_f.to_csv("sample1.csv")
  </code></pre>
  
  <p>As long as I know from the information about reindex function, missing holes should be filled with something (default is "NaN"). I can't find the reason why missing hole in my result files are not filled.</p>
  
  <p>Any idea or comment will be really appreciated.</p>
  
  <p>Thank you,
  Isaac</p>