Not all staff want students to access performance summary information so by default all exams have it turned off. To release it to students the paper owner, or team member, must go into 'Edit Properties' and switch on 'Cohort Performance Report'.
The box and whisker plots are drawn by
draw_boxplot.php which uses the GD library of graphics drawing. The first plot of a line has a scale, this is made by include '&scale=1' as a parameter.
Calculating student marks and therefore whole cohort marks on a paper can be a complicated issue. Marks can be scaled due to standard setting and questions can be excluded post-exam. Performing calculations on the fly would be slow so much of the data used by the box and whisker plots is cached. Class Totals report is the report that fills these caches.
results_cache.class.php class is used to retrieve values from the various caches. Most of the values of the plot are max, min and quartile information. These are obtained by calling
get_paper_cache() with a particular paper ID. The mark of the student is then plotted on the box and whisker diagram; to do this
get_paper_marks_by_student() is called with a student user ID.