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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.
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