Our "Early warning test" (see above) generates a graph like this showing score frequencies for all students:

http://docs.moodle.org/en/Image:Quiz_report_enhancements_historgram.png

It should be possible to produce a graph like this using graphlib.php, and a single database query, so I think we should, and display it after the results table. It should count the number of attempts with a score in each of an appropriate number of score brackets, depending on the quiz total score.