Combine rhubarb, undrained pineapple, sugar, water, and cinnamon candies in a saucepan. Mix well. Cook over medium heat until mixture comes to a boil. Stir occasionally. Combine cornstarch and water. Gradually stir into hot mixture. Reduce heat to low and simmer for 5 minutes. Remove from heat. Stir in salt, lemon juice, and butter. Cool slightly before serving.