It is well-known that Hooke anticipated Newton's law of gravitation. Was this case grave enough such that Newton today would be called a plagiarist?
No. Several people had the idea of an attractive force, and some of them of an inverse square law. Newton worked the idea out in detail, and showed that it produced results that matched the observations of the solar system: he made gravity into a proper scientific theory, rather than a vague hypothesis.
Wikipedia's article on Hooke has a good account of his contributions to the idea.
The story with Hooke--Newton correspondence was well-known at the time of Newton. Newton consciously belittled Hooke's contribution, against the advise of some friends. He also claimed that he had this idea long before Hooke, and this is probably correct. I do not think anyone would call Newton a plagiarist, but his behavior with respect to several contemporary scientists (Hooke, Flamsteed, Leibniz) was rude and arrogant.
Similar things happen frequently in modern times, when some famous and well recognized person fails to acknowledge contributions of other people with not such high standing. This is not usually qualified as plagiarism.
References. The literature on this particular question is enormous. I recommend
Westfall, Never at rest.
The second one is a classical biography of Newton, probably the most comprehensive one, and not surprisingly, the author explains the story from "Newton's point of view". On my opinion, Arnold sees it more from "Hooke's point of view". (I would like very much if someone wrote the story from Halley's point of view).
So you can compare. But nobody calls Newton "a plagiarist". What he did in Principia is by many orders of magnitude more than all his contemporaries together could possibly do. And nobody denies this. But that he was reluctant to give any credit to anyone is another matter.