Marinate cubed chicken in soy sauce for 15 minutes. Cut onion into 1 1/2 inch chunks. Cut pineapple spears into 6 pieces each. Set aside. Lightly oil grill rack and heat grill or preheat oven broiler. In small bowl, combine plum sauce, orange rind, gingerroot and red pepper flakes and mix well and set aside. Alternately skewer chicken, onion and pineapple onto 15 inch metal skewers. Discard leftover soy marinade. Place kabobs 4 to 6 inches over medium high heat or charcoal grill. Brush plum mixture over kabobs. Cover grill and cook 4 to 5 minutes. Turn and brush again. Cover and cook an additional 4 to 5 minutes. To broil skewers, place on broiler pan 4 to 6 inches from heat using above times. Serve over brown rice.