I had the same thought and came looking for this suggestion! Since the graph does count pages & minutes for books that have been partially tracked but not marked finished, I've found it clunky/not intuitive to figure out which book is associated with, say, a lone 50 minutes in May, when I don't remember it could have been. I've tried going to the Reading Journal but it doesn't explicitly highlight the book format there (perhaps this is already suggested in another request/idea) so it instead currently seems to come down to trying to pick out an audio vs paperback/hardback cover image. It's possible to also check "Currently Reading" but, in this case, I had DNF'd the book and forgotten I had done so, making it much more difficult to track down when I became confused by the stats.

For now you can filter your stats page down to March. There you can see which books are marked as read and scroll down to the pages chart to see if there's an outlier. Once you find the date, head to your journal entries and look for the potentially erroneous entry!