You can add less sugar to your recipe, but don't go overboard with the reduction. Start with a 1:1 ratio (1 part sugar and 1 part plums), then next time you make the recipe you can reduce the quantity even more if you want to.   You do need sugar in your plum recipe, especially if your fresh plums aren’t ripe enough. Unripe plums are extremely tart so sugar helps add and intensify the flavor.