I would like to use values of "A, B, C, D.." (grades) instead of numeric
values in a graphic visualisation of a score in different moments in time
(semesters). Can anyone give me a hint or some instructions on how to do this?

ex: if I name the rows math, physics and chemestry, and the columns first,
second, third and fourth semestre, I can easily plot in numeric values like
1, 2, 3, 4.. etc. to get the diagrams to show the right score as a hight of
each column in the diagram. But I would like the diagram to show the results
for 1=f, 2=e, 3=d etc. and be able to plot a, b, c... etc. directly into the
cells. Is this possible? Or is it possible to rename the values of ther
columns in the diagram, so that at least it will apear as a, b, c..?

Thanks for your help!