$(document).ready( function () {

  var table = $('#example').DataTable({

    order: [[0, 'desc']],  // Sort by index column

    columnDefs: [

      {

        targets: 0,

        orderData: [2, 0]  // Sort first by the diff column then by the index

      }

    ],

    rowGroup: {

      dataSrc: [0], 

    },

    initComplete: function () {

      var api = this.api();

      // Loop throug the unique rowGroup.dataSrc values

      api.column(0).data().unique().sort().each( function ( group, j ) {

        console.log(group);

        var diff = 0;

​

        // Get the values for the group

        var values = api

        .rows( function ( idx, data, node ) {

          return data[0] === group ?

            true : false;

        } )

        .data()

        .pluck(1)

        .toArray();

​

        console.log(values);

​

        // Calulate the difference if there are two values

        if (values.length === 2) {

          diff = (values[0] * 1) - (values[1] * 1);

        }

​

        // Get the rows() api for each row in the group

        var rows = api

        .rows( function ( idx, data, node ) {

          return data[0] === group ?

            true : false;

        } );

​

        // Use the rows API to update the hidden column

        rows.every( function ( rowIdx, tableLoop, rowLoop ) {

          var data = this.data();

          data[2] = diff;

          this.data( data );

        } );

      } );

​

      api.draw();

​

    }

  });

​

} );

​