There might be a way to do this, albeit not the simplest.
As you noticed you can add a calculated item to the courses with the post test minus pretest grade.
I believe you may set that calculated grade to be the final determining course grade (should be on the Grades) link in the course. You'll need to repeat this for each of the courses.
Then use the report builder to collect and display all course final grades for tge users which are in effect the gap grades.
Regarding graphical reports, we are wirking on adding graph capability to report builder and actually got a POC prototype working already, but it will take some time to iron it out.
Amir
As you noticed you can add a calculated item to the courses with the post test minus pretest grade.
I believe you may set that calculated grade to be the final determining course grade (should be on the Grades) link in the course. You'll need to repeat this for each of the courses.
Then use the report builder to collect and display all course final grades for tge users which are in effect the gap grades.
Regarding graphical reports, we are wirking on adding graph capability to report builder and actually got a POC prototype working already, but it will take some time to iron it out.
Amir