Using CloudWatch as a datasource for annotations

I’m trying to figure out how to use CW as a source for annotations. I’ve got a Lambda function that I’d like to be a vertical annotation every time that it fires. The only knobs I see in the UI push me towards using CW metrics, which I’ve tried to use to no avail. The Invocations count goes to 1 every time the Lambda runs, otherwise there is no data. Perhaps I’ve got a fundamental misunderstanding of how this is supposed to work.