Heat olive oil in a Dutch oven over medium-high heat. Add onion; sauté 2 minutes. Stir in curry and allspice; cook 1 minute. Add water, broth, sweet potato, and salt. Cook 25 minutes or until potato is tender. Place half of sweet potato mixture in a blender; process until smooth. Repeat procedure with remaining sweet potato mixture. Return puréed sweet potato mixture to pan. Bring bisque to a boil; remove from heat. Stir in 1 cup yogurt until blended. Top each serving with about 1 tablespoon yogurt; garnish with chives, if desired.