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