title: DAX weighted average based on another dimension
body: <p>I have the following data:</p>
  
  <pre><code>DATE        COUNTRY     ITEM        Value
  2005-01-01  UK          op_rate     30%
  2005-01-01  UK          proc        1000
  2005-01-01  UK          export      750 
  2005-01-01  ITA         op_rate     45%
  2005-01-01  ITA         proc        500 
  2005-01-01  ITA         export      350
  </code></pre>
  
  <p>Basically, data in normal format, which includes both ratios (the op_rate) and other items such as exported volumes and processed volumes ("proc").</p>
  
  <p><strong>I need to aggregate by SUM for "proc" and "export", but not for the "op_rate", for which I need a weighted average by "proc".</strong></p>
  
  <p>In this case the aggregated op_rate would be:
  0.45*500 + 0.30*1000 = 0.35 // instead of a .75 SUM or 0.375 AVERAGE</p>
  
  <p>All example I find for weighted average are across measures, but none covers using other dimensions.</p>
  
  <p>Any help most welcome!</p>