As a guy who does a lot of scientific data analysis and plotting, I'm surprised that no one brought up the obvious explanation; in the general scrum of final manuscript preparation, the data analysis guy mistakenly made a plot with the same data replicated several times, but shifted each time so that the plot looked plausible at first pass. It would require a lot of things to go wrong at once, sure... but with experience, you learn to count on a lot of things going wrong at once, especially just before something like a manuscript submission. :)

But that the authors, who presumably could check the raw data the plot is coming from, didn't immediately come back with this explanation, makes it sound much more likely that this is just fraud. Oh well.