Have you had a look at Scapple? It seems ideal for this.
It's not free, but you do get a 30-day trial to see if it's for you. I can't vouch for it myself, but I am using their other software Scrivener for a writing project, and that's an extremely polished and versatile piece of software.
It's not free, but you do get a 30-day trial to see if it's for you. I can't vouch for it myself, but I am using their other software Scrivener for a writing project, and that's an extremely polished and versatile piece of software.