I was randomly searching online because I was helping a friend look for material for her assignment on the Heavy Metal Genre.
I recommended that she watch Global Metal and Metal: A Headbangers Journey by Sam Dunn and Scot McFadyen. Both give really good insights into the world of Heavy Metal.
But What I really thought was awesome was the fact that in the documentary Metal they had a family tree for the Metal Genre. I finally found it and here is it is!