This is a common myth but it is untrue. Diet can affect your acne. Two problematic studies that were done in the 60s and 70s found that chocolate was not a cause of acne. These studies were widely read and accepted so not many other studies were conducted.  Today researchers have found that certain foods can make your acne worse. Some foods include: chocolate, bread, green tea, milk, white sugar, ripe banana, ice cream, apple, orange, and red meat consumption. 
 Jennifer Burris, MS, et. al. Acne: The Role of Medical Nutrition Therapy, March 2013Volume 113, Issue 3, Pages 416–430
 Karadağ AS, et al. The effect of personal, familial, and environmental characteristics on acne vulgaris: a prospective, multicenter, case controlled study from Turkey. G Ital Dermatol Venereol. 2017 Jul 11.