vba program

i have a file in excel (reporty card) where i rate the performances of students based on a test.i want to write a vba program so that individual student's performance is depicted in a chart from the data given.the dates on which student's did not appear for the test shows n/a and it should not show in the chart.i have written the code in the attached file sheet 1
but not getting the desired results.

Use a formula of =NA(), not a value of n/a.

i tried it but not getting the desired results.

Can you explain what that gives you and what is wrong with that as it seemed good to me.