title: python pandas dataframe resample.last how to make sure data comes from the same row
body: <p>I have a <code>dataframe</code> looks like this ,</p>
  
  <pre><code>df = pd.DataFrame({'col1':range(9), 'col2': list(range(6)) + [np.nan] *3}, 
      index = pd.date_range('1/1/2000', periods=9, freq='T'))
  
  df
  Out[63]: 
                       col1  col2
  2000-01-01 00:00:00     0   0.0
  2000-01-01 00:01:00     1   1.0
  2000-01-01 00:02:00     2   2.0
  2000-01-01 00:03:00     3   3.0
  2000-01-01 00:04:00     4   4.0
  2000-01-01 00:05:00     5   5.0
  2000-01-01 00:06:00     6   NaN
  2000-01-01 00:07:00     7   NaN
  2000-01-01 00:08:00     8   NaN
  </code></pre>
  
  <p>and when I perform <code>resample</code> by method <code>last</code>,</p>
  
  <pre><code>df.resample('3T', label='right', closed='right').last()
  Out[60]: 
                       col1  col2
  2000-01-01 00:00:00     0   0.0
  2000-01-01 00:03:00     3   3.0
  2000-01-01 00:06:00     6   5.0
  2000-01-01 00:09:00     8   NaN
  </code></pre>
  
  <p>As can be seen above, the <code>6th minute</code> row has data on <code>col1</code>, so after resample, <code>col1</code> is filled with data on <code>6th minute</code> row, but <code>col2</code> is filled with <code>5th minute</code> row, is there is a way to make sure both data after resample come from <code>6th minute</code> row, that means if <code>col1</code> has data, the resample will not fill <code>col2</code>'s <code>NaN</code> with last, but simply leave it as is?</p>
  
  <pre><code>Out[60]: 
                       col1  col2
  2000-01-01 00:00:00     0   0.0
  2000-01-01 00:03:00     3   3.0
  2000-01-01 00:06:00     6   NaN  &lt;--- if there at least one col has data,the whole row will be used in resample
  2000-01-01 00:09:00     8   NaN
  </code></pre>