Further to my above post I've found out a few things.
1) Grader preferences are per-user and stored in mdl_user_preferences.grade_report_grader_collapsed_categories.
2) The data is serialised so it could look something like this:
...which makes my brain hurt! I want to be able to manipulate this code directly, to 'bulk' collapse a gradebook's categories. Is there someone who has had experience of this who can point me in the right direction? I don't want to reinvent the wheel.
Clarification of default views in the Gradebook
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