As discussed in this forum -> http://moodle.org/mod/forum/discuss.php?d=161046

When a sub-category is hidden, followed by a non-hidden category, the user report is almost unreadable, but only for the actual student. A teacher viewing a student's user report does not see the glitch. An example category structure:

-Category 1 (not hidden)
--Sub-category (hidden)
-Category 2 (not hidden)

I'm fairly sure that the code is not excluding the hidden sub-categories from the initial "children count" when working out what the rowspan should be for a category. Somewhere in grade/report/user/lib.php the rowspan for the category is calculated (function inject_rowspans) but it needs to take into account any hidden sub-categories and avoid inflating the count.

I know display issues are often considered trivial, but I'd regard this as pretty urgent due to the severity of the messed up report. At our college, we use hidden subcategories quite frequently, and would love this to be fixed.