Hi John

It is possible to count the number of uses that have completed a course by creating a report on the Course Completions report source  and adding the course and fullname columns.

Aggregate the fullname column with count unique values 

This should provide a count of users who have a course completion record for each course.

The process of creating a graph is simllar process to the Course Membership Donut but you are choosing completions and pie chart type.

Regards