Even if we assume the graph is nuclear-weapons related, the plot would simply be showing that the bulk of the nuclear fission yield is produced in a short, roughly 0.1 microsecond pulse. This has been standard knowledge since the 1950s. It is not a secret, nor indicative of a nuclear weapons program.
And it turns out there are other problems with the graph, including the shapes of the curves indicating either a sloppy scientific analysis in Iran, or a shoddy hoax by “a country critical of Iran's atomic program”. And, seriously, if it is Iranian – and their scientists are this bad – we probably have little to worry about.
The second AP report, which attempted to walk back their original story, states the possibility that the scientist who allegedly made the graph, Majid Shahriari, simplified it on purpose for presentation to government officials, making serious errors in the process. Even if this unlikely version of events is true, it is possible that Shahriari may only have been pitching an idea to the government to possibly start weaponization work in Iran. But the article goes on to say that “Shahriari was assassinated two years ago.” Even if we give the benefit of the doubt to this version of the story, how is Tehran now supposed to explain to the IAEA a flawed presentation made by a dead scientist who may have been pitching his own personal ideas?