title: Slice with two levels of aggregation. Is it possible over the same table?
body: <p>I'm evaluating Superset and I want to try to understand if it's possible and how to get different levels of aggregation over the same set of data. I'm working with banking accounts. I'm taking daily snapshots of account's balances, so I have a table with the following structure (very simplified):</p>
  
  <p><code>account_id date        balance week
  1           2018-01-01  $5.00   1
  1           2018-01-02  $10.00  1
  1           2018-01-03  $10.00  1
  1           2018-01-04  $10.00  1
  1           2018-01-05  $10.00  1
  1           2018-01-06  $10.00  1
  1           2018-01-07  $10.00  2
  1           2018-01-08  $10.00  2
  1           2018-01-09  $10.00  2
  1           2018-01-10  $10.00  2
  1           2018-01-11  $20.00  2
  1           2018-01-12  $20.00  2
  1           2018-01-13  $20.00  2
  1           2018-01-14  $20.00  3
  2           2018-01-09  $70.00  2
  2           2018-01-10  $70.00  2
  2           2018-01-11  $70.00  2
  2           2018-01-12  $70.00  2
  2           2018-01-13  $5.00   2
  2           2018-01-14  $5.00   3</code></p>
  
  <p>First, I need to calculate the total balance per day. That's pretty straight forward. I can build a Slice using date as Time column and the metric <em>SUM</em> of balance. Using the example data above the results must be:</p>
  
  <p><code>date      total balance week
  2018-01-01 $5.00         1
  2018-01-02 $10.00        1
  2018-01-03 $10.00        1
  2018-01-04 $10.00        1
  2018-01-05 $10.00        1
  2018-01-06 $10.00        1
  2018-01-07 $10.00        2
  2018-01-08 $10.00        2
  2018-01-09 $80.00        2
  2018-01-10 $80.00        2
  2018-01-11 $90.00        2
  2018-01-12 $90.00        2
  2018-01-13 $25.00        2
  2018-01-14 $25.00        3</code></p>
  
  <p>Then I need to calculate the weekly average balance per account. This is easy too, because I can use <em>AVG</em> as metric:</p>
  
  <p><code>week average_balance_per_account
  1     $9.17
  2     $32.08
  3     $12.50</code></p>
  
  <p>But after that, I want to calculate the average total balance per week. That's the average of daily total balance. In this example:</p>
  
  <p><code>week average_balance expression
  1     $9.17           = (5+10+10+10+10+10)/6
  2     $55.00          = (10+10+(10+70)+(10+70)+(20+70)+(20+70)+(20+5))/7
  3     $25.00          = ((20+5))/1</code></p>
  
  <p>Is there any way to calculate this last report using the same table defined before?</p>