Look at example 3 (Multi-column aggregate) here JavaScript Grid: Aggregation | AG Grid. Another use case is a weighted average rather than a simple average.
In my custom aggregate case I would sum a column of ‘daily sales’ of all row nodes to get a ‘grand sum’.
Then I would grab the sum aggregate of ‘daily sales’ column for the current group and divide it by the ‘grand sum’ to get a ratio of the group daily sales to the grand sum.
Then multiply by 100 for a percent - which is the result passed back to the grid for display.
I will put in a feature request, good suggestion, maybe good for ng2 some time in the far future…whenever more votes come in I suspect.
I did add an extra column i the dataset and I did do an InnerRenderer to get the same result - but of course that is only good for the grid option of groupUseEntireRow =true.
It works as is a 'reasonable" UX but not the best since the user could unhide that grand sum column and wonder what is this doing here? {There is no way to permanently hide a column from view but allow it for calculations).