To prepare sauce, combine first 4 ingredients in a small saucepan (A); bring to a boil. Reduce heat; simmer 25 minutes or until thick, stirring occasionally (B). Remove from heat; stir in 1/8 teaspoon salt. Pour into a small bowl; cool to room temperature.
Prepare grill to medium-high heat.
To prepare the chicken, sprinkle the chicken with 1/8 teaspoon salt and pepper. Place chicken on a grill rack coated with cooking spray. Grill 4 minutes on each side or until done. Serve chicken with plum sauce.