ISP can view month wise invoice payment total amount as a piece of summery information.to view a summary information on sidebar Billing > Summary option.
now you can show a page on Invoice Payment Month wise total amount in graphical way Chart.it will by default show a current year all month to change then go on card header select year option and change year as you want.
to view an in which month how many amounts generated in invoice and payment then just live your mouse on a chart that will show a tooltip small box it will contain month name year and amount total.
