In a large saucepan, combine the rhubarb, 1 cup water, sugar and cinnamon; bring to a boil. Reduce heat; simmer, uncovered, for 15-20 minutes or until rhubarb is tender. Combine cornstarch and remaining water until smooth; stir into rhubarb mixture. Bring to a boil; cook and stir for 2 minutes or until thickened.
Remove from the heat and discard cinnamon stick. Stir in grape juice. Serve warm or cover and refrigerate overnight. Garnish with almonds if desired.