We had a glut of purple plums in the past - SW France. We made mostly compote. No exact measurements, but as far as I know the basic fruit chutney recipe is finely chopped onions stewed in cider vinegar with spices. Then add the fruit and cook gently until thickened. With some fruit you need sugar but our ripe plums were already very sweet. Preparing the plums, or other fruit, is the longest job. Removing the stones and chopping the fruit. If you live in a warm climate an alternative is to lay them out in the sun and make prunes. As they did where we were.
Cut the plums in half and then cook with the stones in. They will float to the top and you can skim them off without any hassle.
Be generous with the spices - cloves, star anise and black peppercorns, even a few chilli flakes, all go well with plums. Put whole spices in and then fish them out at the end. Brown sugar is better than white.