CANTON, MASS. —Dunkin’ Donuts, the 7,200-unit chain based here, augmented its popular breakfast sandwich line with the launch of the Maple Cheddar Sandwich. The sandwich, which will be available systemwide through March 11, combines scrambled egg with a maple-flavored sausage patty topped with melted white cheddar cheese and served on a croissant. The sandwich is available throughout the day for a suggested price of $2.99. Chainwide sales of breakfast sandwiches rose more than 10 percent during 2006, the company said.