I think lumping "novel" adaptations together ends up confusing things quite a bit especially if you go back a few more decades that the graph and don't just start in 2000.

Classic western novels like "A Dog of Flanders" and "Heidi, Girl of the Alps" or more contemporary western novels like "Howl's Moving Castle", don't really fit into the same category of Japanese light novels like KonoSuba or Haruhi Suzumiya or even more classic ones like Slayers.

Adaptation of Japanese novel series are usually much more direct and true to the original while outside sources tend to be far more lose in their adaptation.

There also is a much bigger trend of franchises doing anime, manga and light novels series all at once while the original is still being written, while outside sources are far more removed for the original.