title: dataframe multiply some columns with a series
body: <p>I have a dataframe df1 where the index is a DatetimeIndex and there are 5 columns, col1, col2, col3, col4, col5.</p>
  
  <p>I have another df2 which has an almost equal datetimeindex (some days of df1 may be missing from df1), and a single 'Value' column.</p>
  
  <p>I would like to multiply df1 in-place by the Value from df2 when the dates are the same. But not for all columns col1...col5, only col1...col4</p>
  
  <p>I can see it is possible to multiply col1*Value, then col2*Value and so on... and make up a new dataframe to replace df1.</p>
  
  <p>Is there a more efficient way?</p>