The following diagrams try to show many of the factors and interactions in the series, to see how various choices of the characters affected other characters. These diagrams do not show ALL the factors involved; they concentrate mainly on Dumbledore, Tom Riddle, Snape, and Harry.
Part 1: Dumbledore and Grindelwald and Tom Riddle
Part 2: Dumbledore and the Marauders and Snape
Part 3: Harry At Hogwarts Year 1 - 5
Part 4: Half-Blood Prince and the Deathly Hallows