I know what you're talking about, but have no idea why it is there for some quizzes and not for others. I don't usually find it very informative, anyway, so just glance at it when it is there. It may be that there is a minimum number of plays required before the graph is generated (too little data makes ugly graphs), not just a time issue.
(Editor in Humanities, Religion, Literature and Books For Children)
That's all, folks!