Who is "Portrait of Tracy" by Jaco Pastorius about?

I've been listening to this clic instrumental by bist Jaco Pastorius alot lately. I was wondering if anyone knew who Jaco wrote it about, or maybe who "Tracy" even was? Was he/she an actual person? Someone he created? Maybe just someone he had in mind...? It's been digging on my mind more than usual these days. The song's grown on me and it'd mean so much if someone could help me out. THANKS!