How do I create a report that tells me which courses a user has completed?

Sorry guys, yes. This is often called a transcript. My understanding is that it is expected as a new feature in July. You can get the specific reporting data for each course using the by course detailed reports, but this is essentially aggregated reporting, then filtered by individual learner. That's expected in the transcripts. You can get the aggregate data filtered by other fields, eg. Manager, job role, even by course - but it isn't yet available by individual learner - and probably the chart format isn't what you'd want here anyway - you'd likely want something more like a formal transcript, to more easily see the information. Below are a couple of shots of the aggregate data from reporting by completions in case that's of some help.

Doesn't look like you ever got an answer to this. I had the same question and learned that this will be addressed in a future release expected in July 2016.