Trouble calculating column sum on view with active views group_by (aggregation)

I second the need for this feature, because it greatly improves performance if grouping is done by the database. Patch #2 works well, is there any reason not to include this in the module, at least as a configurable option so existing views do not break?

This is a reroll of patch #2 against 2.1.x

Also, see #3322452: Warning: Undefined array key XXX in Drupal\views_aggregator\Plugin\views\style\Table->getCellRaw() → which makes a very similar change for a different reason.