Inserting rows in middle of table

msand01

I am attempting to insert a row in the middle of my datatable, utilizing the first column as the sort order - eventually i would like to hide this column. It appears to work initially; however subsequent inserts (or deletes) reveal (I suspect) that the underlying cache hasn't in fact been updated, since the rows no longer appear in the correct order. I am obviously missing something but have been unable to fix it myself.
I have an example here: http://live.datatables.net/jekugolu/1/edit

msand01

Maybe this is not possible?

It does appear to work, but if you try to delete a row after adding, the added rows go back to the bottom.

Here is the pertinent code code from the above live site:

        testTable = $('#test-table').DataTable({
            "data": dataForDataTables,
            "destroy": true,
            "pageLength": 25,
            "columns": [
                { "data": "SortIndex" },
                { "data": "DataGroup", "orderable": false },
                { "data": "Number1", "orderable": false },
                { "data": "Number2", "orderable": false },
                { "data": "Label1", "orderable": false },
                { "data": "Label2", "orderable": false },
                { "data": "Delete", "orderable": false, "render": delete_icon },
                { "data": "Insert", "orderable": false, "render": add_icon }
            ]
        });


        $('#test-table tbody').on('click', 'i.glyphicon-plus-sign', function (event) {
            var insertedRowIndex = testTable.row($(this).parents('tr')).index();
            addRow();
            var lastRowIndex = testTable.page.info().recordsTotal - 1;

            //re-number sortIndex column to reflect added row 
            $.each(testTable.rows().data(), function (i, row) {
                var firstCell = testTable.cell(i, 0);

                if (i < insertedRowIndex) {
                    firstCell.data(i);
                }
                else if (i >= insertedRowIndex && i != lastRowIndex) {
                    firstCell.data(firstCell.data() + 1);
                }
                else if (i == lastRowIndex) {
                    firstCell.data(insertedRowIndex);
                }
            })
 
            testTable.order([0, 'asc']).draw();         
        });

function addRow() {
    testTable.row.add({
        "SortIndex": testTable.page.info().recordsTotal,
        "DataGroup": $("#select-data").val(),
        "Number1": "",
        "Number2": "",
        "Label1": "",
        "Label2": ""
    }).draw();
}