I don't see why you need more than one series for this. Exactly what type of chart did you have in mind? If you make the employee's name or ID the x-axis and their score the y-axis, you can chart varying numbers of employees just by which records you include in your found set.
A normal line chart, each person with their own line. y axis is points, x axis is dates.
I don't think you can get a line chart like that when the number of y-series data sets are not known in advance in Filemaker. You may need to either use a different chart type or use another tool for charting the data.
I haven't done this myself, but I've seen web viewers with Java used to chart data in ways not possible to do with a built in FileMaker chart.
And you can also export the data to a different tool for purposes of generating the chart--you might then be able to import/insert an image of the resulting chart into a filemaker container field if such be useful to you.