Commits on Mar 6, 2020

1. Reduce the roundoff error in the Avg of N others calculation. …

   The "Average of N others" calculation was performed using the
   AVG() function in SQL then multiplying by the number of rows and
   dividing by the total number of data series. Since AVG() is
   SUM()/COUNT() the calculation was:
   ```
      SUM()         COUNT()
    ---------   *  ---------
     COUNT()         total
   ```
   
   Performing the calculation in this way increases the roundoff error.
   This pull request changes the code to instead calculate the average as
   ```
      SUM()
    ---------
      total
   ```
   which has lower roundoff error due to fewer operations.
   
   The keen eyed reader might ask why not do the calculation in SQL rather
   than the SUM() in SQL and division in PHP. This is to maintain backwards
   compatibility with the existing behaviour where if the average value is
   zero then it is plotted on the chart as a zero. This differs from the
   rest of XDMoD where zero values are not plotted. If the calculation were
   preformed in SQL then a zero would be converted to null in the
   `convertSQLtoPHP()` function. This, of course, could be changed too, but
   this would be a much more intrusive change.

   

   jpwhite4 committed Mar 6, 2020
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