1. Combine first 5 ingredients in a small bowl; stir in 1/8 teaspoon salt. Scrape mixture into a zip-top plastic bag. Add chicken; seal. Marinate in refrigerator 2 hours, turning after 1 hour.
2. Immerse 12 (8-inch) wooden skewers in water; soak 30 minutes. Drain and pat dry.
3. Preheat the grill to medium-high heat.
4. Remove chicken from marinade; discard marinade. Thread 3 chicken pieces, 1 pepper piece, and 1 tomato alternately onto each skewer, beginning and ending with chicken; sprinkle evenly with remaining 1/2 teaspoon salt. Arrange skewers on grill rack coated with cooking spray; grill 4 minutes on each side or until done. Serve with yogurt.