Waldorf Apple Salad

2-3 stalks celery, finely diced
1 granny smith green apple, diced
2 red apples, diced
1 cup grapes, halved
1/2 cup raisins
1/4 chopped pecans, optional

Dressing:
1/2 cup plain yogurt
1/4 cup mayonnaise
1/4 cup buttermilk (or sour cream+milk)
2 Tbl. sugar (to taste)