BASE: Combine crumbs and margarine; press onto bottom of 23 cm springform pan. Bake at 350, 10 minutes. BODY: Combine cream cheese, sugar and banana, mixing at medium speed on electric mixer until well blended. Add eggs, one at a time, mixing well after each addition. Stir in walnuts; pour over crust. Bake at 350, 40 minutes. Loosen cake from rim of pan; cool before removing rim of pan. TOPPING: Melt chocolate pieces and margarine with water over low heat, stirring until smooth. Drizzle over cheesecake. Chill.