Preheat your barbecue or indoor grill to medium-high. Place the first 7 ingredients in a pot and bring to a simmer. Mix in the plums, cover, and reserve sauce on low heat until the chicken is cooked.
Brush chicken with the 2 Tbsp. oil; season with salt and pepper. Lightly oil the bars of the grill. Grill the chicken 3 to 4 minutes per side, or until cooked through. Plate the chicken. Spoon the sauce over the chicken, sprinkle with mint and serve.