Hello @Wieger Kloppenburg,
Thank you for bringing this topic to the discussion.
If I understood correctly, you just want to add the aggregates below the data just like Wijmo is doing right now - here.
I attached a sample to this message. You can take a look if you want to get a better look at what I am going to explain.
To do so, you will need to specify your grid's name (whatever you want to name it):
Then, in the preparation of the Web Screen, you will need to add a new Server action - in this case, you will need to use the Javascript action:

Now it's time to prepare the method addSumRow.
You can do that by adding the method into the Javascript of the Web Screen.

Check the code below:
var addSumRow = function(gridId){
var gObj = GridOS.ComponentUtils.getGridObjectById(gridId);
var grid = gObj.grid;
grid.columnFooters.rows.push(new wijmo.grid.GroupRow());
grid.bottomLeftCells.setCellData(0, 0, 'S');
};
This should be enough to customize your bottom row with the aggregation values from the numeric columns.