Mix sauce up- stir together sugar, vinegar, water, soy sauce, cornstarch. Set aside
Preheat wok over high heat; add oil. Stir fry onion for 1 minute, add chicken and stir fry until done, about 3 minutes. Push mixture from center of wok. Stir sauce mixture and pour into center of wok. Cook and site until thick and bubbly. Add vegetables and ginger, stir fry for 2-3 minutes more. Stir in plums. Serve over rice or noodles.