Rowling has revealed information and stories about the wizarding world in other formats besides the novels. All (or most) of the information from these sources can be verified as being created by Rowling, so we consider such information to be canon.
The other sources used by the Lexicon are:
Rowling’s writings on Pottermore
Pottermore Presents ebooks
Wizarding World of Harry Potter
Rowling’s handwritten planning document for Order of the Phoenix
Rowling’s hand drawn illustration for Dragon Breeding for Pleasure and Profit
Rowling’s hand-written “idle jottings” circa PA with names of teachers and hippogriffs
Harry Potter and the Cursed Child
Fantastic Beasts and Where to Find Them (film series)
Fact sheets from the Bloomsbury Children’s Editions of the novels
What was the Name of that Nymph Again? or Greek and Roman Studies Recalled