As has been noted previously, the character Joan Gilling is loosely based on a real person, Jane Anderson, who was a contemporary of Sylvia Plath's at Smith College.
As has also been noted on several occasions, The Bell Jar is full of fictionalisations of easily recognisable individuals and events, and it was this aspect of the novel which led to specific difficulties in relation to the fictionalisation of Jane Anderson:
In 1987, Anderson (a doctor specialising in psychiatry) brought a defamation case against the makers of a 1979 film based on The Bell Jar, claiming that the portrayal in the film of lesbian tendencies in the character Joan Gilling, and the fact that she hangs herself, had damaged her reputation.
The case inevitably highlighted the enormous complexity that arises when real-life events and real individuals are fictionalised. This complexity is neatly captured by Jacqueline Rose in her book, The Haunting of Sylvia Plath (p.108), when she states: Anderson's case rested, necessarily, on a double claim - that she both was, and wasn't, the character portrayed in the novel and the film.
To learn more about the ramifications of this case, click here.