How to declare synthetic column in Table View?

Hello,
Use case if of synthetic column in view. A synthetic column is an extra column whose cell value s a function of existing column values.
Based on limited understanding of column.render function, defined a column as follows:

   'data': null, title: 'gain/loss', render;{'type':'number', 'display':'xxx'} . 

Hoping that  'xxx' would be replaced by a function to calculate 'gain/loss' column value ti display

However, with this setup, DataTable warns with familiar warning :
'DataTables warning: table id=data-table - Requested unknown parameter 'null' for row 0, column 7. For more information about this error, please see http://datatables.net/tn/4'

Once we go further from that warning, the browser console shows error

at jquery.dataTables.min.js:67 'Uncaught TypeError: can not read property 'style' of undefined' .

Any help on how to define synthetic column of table view?

Regards --