Each line on this chart follows a major set of characters. The one beginning at "Colour of Magic" involves Rincewind and the wizards.

The one starting at "Equal Rites" involves Granny Weatherwax and the witches. The last two, "Wee Free Men" and "Hat Full of Sky" are (theoretically) juveniles starring a girl named Tiffany, with the witches only coming on in cameos. The next novel, I believe, falls into this category.

The sequence starting with "Mort" is basically Death and his relatives, especially his granddaughter.

The one starting with "Guards, Guards!" is the Watchman series, starring Vimes, a crusty Watchman born to be played by the younger Clint Eastwood (if he was British.) This series is the most politically interesting. "Night Watch" was the winner of the 2003 Prometheus Award for libertarian fiction.

The other ones off to the sides, like "Small Gods," take place in the Discworld universe, but are pretty much standalone. Characters from the other series do appear. In fact, characters cross over from one group to another all over the place.