Share prices of the fast-casual restaurant chain have plummeted 80% this year.Automation and healthy eating. That combination sounds like it should make for a great investment opportunity. And that's what many Sweetgreen (SG 3.40%) investors were probably thinking when they bought shares of the business in recent years. But despite all the promise surrounding Sweetgreen's business, the results haven't lived up to the potential. This year, the stock has cratered an incredible 80% in value.To crash so signifi ...