For Moodle 2.2.2 I found a way to add a dropdown list of the sections/weeks of a course on the "Grader report" (Gradebook) page and then display only the Gradebook items for the selected section/week. The main advantage of this approach is that little or no horizontal scrolling is needed to use the Gradebook when the Teacher is grading just one section/week at a time.
This uses one of the data structures already built in the Gradebook code to determine which section/week is associated with a grade item and which CSS class is used for its column of the Gradebook display. It filters the display of Gradebook items for the selected section/week by applying (1) the YUI hide() function to all cells that should not display and (2) the YUI show() function to all cells that should display.
There are limitations to the current version of my code:
(1) A course with many "manual" Gradebook items, which are not associated with any section/week, would not benefit much from this mechanism. Instead, one would need to display the entire Gradebook rather than just the items for a selected section/week to grade the "manual" items.
(2) This is _not_ implemented as a separate plug-in, but instead changes some of the core Moodle code directly. It will require maintenance each time we upgrade to a newer version of Moodle.
(3) The current version of the code assumes that the option "Static students column" ("grade_report_fixedstudents") is checked at: "Site administration > Grades & Attendance > Report settings > Gradebook". This option ensures that Gradebook displays the student name in a fixed column on the left while the other Gradebook items to the right can scroll horizontally. If that checkbox is not checked, the code will not select the correct CSS classes to hide and show.
(4) There may be other course variations that it does not support, even if it supports all the courses on my Moodle site. For another example of how different course settings can affect the code, the data structure for a Gradebook containing multiple categories differs from the data structure for a Gradebook containing only one category, so the code handles those cases separately.
If this sounds like a useful approach toward making the Gradebook more user-friendly, I will clean up my code so that it is less embarrassing and everyone will be welcome to it. The patch files still total less than 200 lines, so it is not a large amount of code.