title: Double aggregation in Tableau using LOD expressions
body: <p>I am using Tableau to create a custom google analytics dashboard. I have a custom dimension named <code>author</code> in my google analytics view and I would like to group the date of the first page/view by author and by month having a counter.
  I successfully get the date of first page/view using <code>MIN([Date])</code>, but I don't figure out how to use LOD expressions to double aggregate a calculation. I tried the following expression, but tableau shows an error saying that the argument I'm trying to count is already an aggregation and can no longer be aggregated. </p>
  
  <pre><code>{INCLUDE [Author] : COUNT(MIN([Date]))}
  </code></pre>
  
  <p>What did I miss ?</p>