To prepare glaze, place grapes in a blender; process until smooth. Heat 2 teaspoons oil in a saucepan over medium heat. Add onion; cover and cook 10 minutes. Add garlic; cover and cook 3 minutes, stirring occasionally. Stir in pureed grapes, vinegar, soy sauce, sugar, and 1 teaspoon rosemary; bring to a boil. Reduce heat, and simmer 10 minutes or until slightly thick. Cool slightly. Place grape mixture in blender; process until smooth. Set aside.
To prepare chicken, brush 1 tablespoon oil over chicken; sprinkle with 2 teaspoons rosemary, pepper, and salt. Place chicken on grill rack coated with cooking spray; cover and grill 25 minutes or until done, turning and basting frequently with grape glaze. Garnish with rosemary leaves, if desired.