Rhubarb jam is a joy to make but its tartness and wateriness when you boil it means you need to have a 50/50 mix of fruit to sugar. This means you can end up with a very sweet result. The 2 way around this is to add plenty of lemon juice at the end of cooking or add something like ginger, the heat distracts from the sweetness, or as in the recipe the bitter sweetness of grapefruit.
These rhubarb have come from my garden in Peckham. This is their 3rd year and they really enjoy the shelter of the brick wall of the railway line at the end of the garden
2kg rhubarb, picked, washed and chopped
2kg sugar jam
1 white grapefruit, large