grade display in gradebook can not be customized

The gradebook displays two columns to the left of the student's name: Total and Average. The Total column should be removed, as it is meaningless in any but the most trivial grading situations (weighted grade categories, for example, would render the value meaningless).

I want the Average column replaced by a "Current Grade" column that could have display A through E instead of a percent. I want to compute grades this quarter based on a 4 to 0 scale, where 4 is A, 3 is B, etc.

Here is how it looks now:

http://ict.gctaa.net/~jelkner/images/gradebook_before_fix_270409.png

Here is how I would like it to look:

http://ict.gctaa.net/~jelkner/images/gradebook_after_fix_270409.png