Line chart plotting exam grade letters (y-axis) against date (x-axis)

My high school students receive exam grades in the format A, B, C, D, etc. I want to be able to plot these for each term in our school calendar to show their progress across the year. I have been able to make this work by assigning a number to each grade (eg 50 for an A, 40 for a B, etc) so that the line chart shows the variation in their grades across the year, but I can't get it to work with the grade letters themselves.

Here's what I have achieved so far:

I have tried adding a table that shows the relationship between the numbers I assigned and the grade letters, but this doesn't help. I want grade letters on the y-axis instead of the numbers shown above.

Any suggestions would be welcome.