Anki add-on: Learning Step and Review Interval Retention

This add-on adds statistics showing the percentage of correct answers for each learning step and review interval. Those can be used to tune learning steps and interval modifier. Retention is shown separately for learning, relearning and cramming reviews. Statistics are calculated from the nominal intervals Anki stores in the review history. This means that same-length learning steps are grouped together and reviews are counted for their scheduled interval even if done early or late. This add-on does not support the new stats in Anki 2.1.28 or later. To access the old stats, hold shift while opening the stats window. Advanced mode: If advanced mode is turned on, a more complex algorithm is used. It tries to separate successive same-length learning steps and show the first review after graduation as a separate step. It can be slow with collections with lots of reviews, and it is likely to miss some edge cases. Version history: 2025-08-21: - Added an option to set the statistics period to any number of days. 2025-04-18: - Improve compatibility with recent Anki versions by ignoring manual and rescheduled entries in the review history. - Added a workaround for the bug in Anki that causes some of the graphs to be empty unless a deck is manually selected after starting Anki. 2024-01-06: - Added an optional graph showing review retention for long intervals. It can be enabled with the hide_review_month option. - Round learning intervals over 1 day to whole days to match the changes in the v3 scheduler. The old behavior can be restored with the round_steps option. For more details, see the support thread on the Anki Forums. 2022-08-28: - Improve compatibility with the V3 scheduler. For more details, see the support thread on the Anki Forums. 2022-03-14: - Support for Anki 2.1.50 - Removed support for translated texts as Anki has not supported them for a while 2020-12-28: - Add advanced mode 2020-09-26: - Show non-integer learning step lengths as minutes and seconds 2020-01-12: - Initial release