Please add a "preferences" tab at the top of the report (grade/report/grader/preferences.php and tabs.php for the actual tabs part)

The preferences are all user preferences that affect how things are displayed to that user, and can be stored using set_user_preferences() and access from $USER later.

The defaults for the settings should be settable in the admin menus (ie defaults for the whole site) stored using set_config()

I'm leaving out the idea of course-wide settings for now.