Simplest way would be to use a found set with a summary field (absolute number in group) and a calculation (absolute number in group / absolute number total)**; then you can draw the values to chart using the Data Source option "Current Found Set: Summarized groups of records".
Don't forget that for the summary field (and your chart) to work, you must sort your records by grade.
Which several things have you tried so far?
**EDIT: the Chart tool can display percentages as an option, so there's no extra field required.
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And here's a very basic sample file attached.
Note that if you only want to display the chart, you don't need a body or sub-summary part; the layout doesn't even have to be in List view.
The only things you really need are a summary field and a sorted found set (and the correct Data Source setting in the Chart tool).
That worked but I am not sure why mine didn't as I did exactly that with the summary field. I will go back and see where I broke down. Anyways thank you for the clarification and the file. Much appreciated.