Agree in that direction you need six columns.
However, that will display the grand totals and not totals just for the displayed periods (months)
In order to display the totals for just for the displayed periods you would have to go the hard way:
Switch to a query based portlet where you can query whatever you want for the total columns.
If you want to be able to change the period and the start of the time scale that is going to be even more painful, but doable.
It is all in the ingenuity of the develeper.