Remove fat from chicken and make 3 or 4 long slits in meat to allow marinade to penetrate. Mix yogurt, 1/4 cup chopped mint, ginger, coriander, cayenne and sugar to make marinade. Divide marinade, reserving half in a separate bowl. Add chicken to one half of the marinade and let marinate 10 minutes, turning once during this time. Heat margarine on medium high in a nonstick skillet just large enough to hold chicken. Remove chicken from marinade; discard marinade. Add onion, garlic and chicken to the skillet. Brown chicken 3 minutes. Turn and brown 2 minutes. Lower heat to medium. Spoon reserved marinade over chicken, cover and cook 5 minutes. Sprinkle with remaining 2 tablespoons chopped mint and serve.