It looks to me like your data should fall into discrete categories for your grades: (90...100 = A, 80..89 = B, and so forth)
That would seem to better fit a bar chart of your data instead of an area chart.
In any case, to answer the specific question asked in your title, with each record recording a different grade, you sort the graph data by sorting the records from which they are charted. But unless you use the "show data points for groups of data" option, each record charts as a separate point as you have already discovered.
When I click "show data points for groups of data" it still treats each record as a separate point.
Not if the records are sorted by grade and if the records have exactly the same grade.
I don't think this will give you the results you want, however unless you also have a summary field to count your records to use for the Y axis.
Even then if you have values like: 85.2%, 86.3%, 98.2%... you may want to set up a calculation field that produces a single value for all scores that fall in a given range (and then you'd sort by this calculation field.)