I have suggested this one before that we rename “true retention” to “retention rate”. In the current context, the word true doesn’t make any sense. And it might also suggest that there’s some sort of “untrue retention” out there.
For some perspective, the add-on that started it all used the word true to mean “retention rate excluding learn state cards”. And as the “again rate” in stats included learn cards it made sense to claim that “true retention” is somehow truer than the “again rate”.
The “rate” in “retention rate” indicates a mathematical (percentage) value about your retention. “Retention” just refers to the psychological process itself.
I vote for “Retention Rate”, since we’re actually talking about a percentage representation.
The chart shows tallies too, not just percentages. If you really want a 2nd word, I usually use “retention results” or “retention outcomes” when I’m talking to users about it (because I also dislike calling it “True Retention”).
But I think a simple title like “Retention” fits in nicely with the rest of Stats – like having a “Reviews” graph that shows you number of cards and time spent, but not calling it “Number of Reviews” or “Time Spent Reviewing”