how to create graphs using letter data

im trying to make a graph which will show a student progress through a certain amount of task currently i have the effort grade per task in a graph but i cannot get the grade per task to work. the grades are in letter format which may be the reason why but i need to find a way to get it to output the graph with the letters others i wont meet my clients expectation of what my system should show. i have attached a spreadsheet and the sheet you will need to look at is the process sheet this has the data which will be used to form the graph