Combine everything in a large pot and bring to a boil over medium-high heat. Reduce to medium low and cook, stirring often, until the rhubarb breaks down and the mixture thickens and turns jam-like. It should take about 45 minutes. To determine whether it's ready, drop a spoonful on a cold saucer (put it in the freezer for a few minutes) - it should look like jam. If you like runny jam, leave it runny - if you like it thicker, cook it a little longer.