Combine ingredients in a sauce pan. Bring to a boil over medium heat. Reduce heat to a gentle simmer until rhubarb and strawberries are tender - just a few minutes. Don't worry if your strawberry & rhubarb mixture is a little runny. The chia seeds will continue to thicken it as it cools. Serve layered on top of plain Greek yogurt for a healthy and refreshing treat. You can also use as jam or as a topping for oatmeal. Keep the strawberry rhubarb mixture in fridge for one week or freeze to keep longer.