title: pandas: read_csv combined date-time columns as index into a dataframe
body: <p>I have a csv file which contains date and time stamps as two of the columns. I am using pandas <code>read_csv</code> to read the contents into a dataframe. My ultimate goal is to plot time series graphs from the data. </p>
  
  <pre><code>!head vmstat.csv
  wait_proc,sleep_proc,swapped_memory,free_memory,buffered_memory,cached_memory,swapped_in,swapped_out,received_block,sent_block,interrups,context_switches,user_time,sys_time,idle_time,wait_io_time,stolen_time,date,time
  0,0,10896,3776872,380028,10284052,0,0,6,16,7716,4755,3,1,96,0,0,2012-11-01,08:59:27
  0,0,10896,3776500,380028,10284208,0,0,0,40,7471,4620,0,0,99,0,0,2012-11-01,08:59:32
  0,0,10896,3749840,380028,10286864,0,0,339,19,7479,4704,20,2,77,1,0,2012-11-01,08:59:37
  0,0,10896,3747536,380028,10286964,0,0,17,118,7488,4638,0,0,99,0,0,2012-11-01,08:59:42
  0,0,10896,3747452,380028,10287148,0,0,0,24,7489,4676,0,0,99,0,0,2012-11-01,08:59:47
  
  
  df = read_csv("vmstat.csv", parse_dates=[['date','time']])
  f = DataFrame(df, columns=[ 'date_time',  'user_time', 'sys_time', 'wait_io_time'])
  
  In [3]: f
  Out[3]:
  date_time               user_time  sys_time     wait_io_time
  0  2012-11-01 08:59:27          3         1             0
  1  2012-11-01 08:59:32          0         0             0
  2  2012-11-01 08:59:37         20         2             1
  3  2012-11-01 08:59:42          0         0             0
  4  2012-11-01 08:59:47          0         0             0
  </code></pre>
  
  <p>So far, we could read the data correctly and <code>date_time</code> is combined in the DataFrame. There are issues if I try to used the <code>date_time</code> from <code>df</code> as index. Specifying <code>index = df.date_time</code> gives all <code>NaN</code> values:</p>
  
  <pre><code>dindex = f['date_time']
  print dindex
  g = DataFrame(f, columns=[ 'user_time', 'sys_time', 'wait_io_time'], index=dindex)
  
  In [7]: g
  Out[7]:
  0    2012-11-01 08:59:27
  1    2012-11-01 08:59:32
  2    2012-11-01 08:59:37
  3    2012-11-01 08:59:42
  4    2012-11-01 08:59:47
  Name: date_time  &lt;---- dindex
  g:
                   user_time  sys_time  wait_io_time
  date_time                                             
  2012-11-01 08:59:27        NaN       NaN           NaN
  2012-11-01 08:59:32        NaN       NaN           NaN
  2012-11-01 08:59:37        NaN       NaN           NaN
  2012-11-01 08:59:42        NaN       NaN           NaN
  2012-11-01 08:59:47        NaN       NaN           NaN
  </code></pre>
  
  <p>As you see, the column values are coming out as all <code>NaN</code>s. How do I get correct values as in the intermediate <code>f</code> frame?</p>