This graph is called Receivables by Aging Period if Sage Simply Accounting is set to use accounting terminology.
Unpaid Sales by Aging Period is a pie chart showing the total amounts your customers owe you for the current period and three aging periods, and the percentage of the total that each period represents.
Use it to see a quick overview of the proportion of your sales invoices due in each period, to find out whether most of your customers are paying you on time.