I'd consider the task to build a diagram understandable without already having in-depth knowledge impossible. For all but Singer and Gilligan their ethical systems are built upon and intertwined with very diverse systems of epistemology which are sometimes not even explicit, making things even more complicated.
It's facing quite a task, indeed. If you want to get in touch I'd rather read up on them in the Stanford Encyclopedia of Philosophy. For most/all of them there will be something on "Freedom", "Morality", or "Ethics" and the search results of "[Author] ethics" will also give you an idea about how to categorize their ethical thought.
Another idea would be the Oxford Handbook of Ethical Theory (available in most university libraries either physically or online), taking the index to find texts where the names are mentioned. This also helps learning about the various labels under which ethical theories are subsumed.
Strategies like these are helpful for all subject matters and I guess you will not find any helpful diagrams for broad philosophical fields. Like, ever. I made some for characterising Kantian ethical theory alone in its various historical manifestations. They were quite complicated already.
FWIW, reading a bit on the authors helps to sort out pretty fast. I have yet to learn about a person not finding their "favourite" ethical theory - usually, the one which aligns best with what they already thought to be right.