I know of ersatz coffee brewed from a range of non coffee ingredients, including acorns, but sweet potato is a new one on me...
If it ain't got Cofveve beans in it, it's TEA, Nuff Said! Want real old school cofveve the RIGHT way, Start with your basic beans in a hand crank grinder, put the grinds in the strainer and add 1 pinch of salt, 1 pinch of pepper, one sprig of crushed chicory root, and one crushed egg shell, and let it steep as it filters through, drink as is, or add your flavorings of choice, ( Gotta add a little sweet cream to mine) and you got REAL cofveve in your mug! Save the grounds for either the garden or add to Black Bread dough for that extra yummy!
As I might have mentioned in the past, I'm a Civil War re-enactor and historian. I have been a student of the War Between the States for 60 years and will be until I die because it is impossible to know everything there is to know about the War of Northern Aggression. During the War, those nasty Yankees cut off the noble South's supply of COFFEE! Talk about your war crimes! However, the people of the South developed substitutes for coffee, these included parched corn, rye, wheat, okra seed, chicory, sweet potatoes, acorns, dandelion roots, sugar cane, rice, cotton seed, sorghum molasses, English peas, peanuts, and beans. The "best" of these seems to have been rye, corn, okra seed and sweet potatoes, however they all lacked one important thing and that was caffeine. After four years of valiant resistance the fighting men of the South were forced to lay-down their arms, and while they were out-numbered, out-gunned and starving, coffee could have changed the outcome of the War.