One inheritance, many hands
This is the diagram the whole portal is arguing for. The lines are the routes along which the material actually travelled, and the point of the exercise is what happens to an object as it passes down one of them.
Pick a motif and watch it change. The sword starts as a Welsh war-leader’s weapon, is latinised and forged in Avalon, is demoted below the Grail, and ends up split in two — a stone that elects a king, and a lake that lends him a blade and takes it back. Nobody corrupted anything. Each of them made it mean something they needed it to mean.
Choose a motif above to trace the route it actually travelled, and see what each hand changed about it.
The dotted ones
One edge is drawn as unresolved: the Welsh tales and Chrétien tell two of the same stories, and a century of scholarship has not settled who borrowed from whom. Treating the Welsh material as straightforwardly the original is a reflex the evidence does not support, and so is the reverse.
One edge is drawn as an absence. Sir Gawain and the Green Knight is the finest poem in the tradition and it left no mark on Malory whatsoever — it survives in a single manuscript and was effectively unread for four centuries. Influence is not a function of quality, and a diagram that only drew the successful transmissions would imply that it is.