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I have a dataframe consisting of a number of different subjects, and the percentages of grades obtained in each of them. I can produce a percentage chart with:

ggplot(data=marks,aes(fill=Grades,y=Percentage,x=Subject)) + geom_bar(position="fill",stat="identity")

However, the letter grades I'm using include HD (High Distinction), D (Distinction), C (Credit), P (Pass), N (Fail) and a few others for withdrawn, ungraded pass etc. The plot command automatically orders them alphabetically: C, D, HD, N, P ... whereas I want them in order of highest grade to lowest: HD, D, C, P, N, ...

How can I include that order in my ggplot function call?

Note, if it helps, the first few rows of the dataframe are:

   Grades Subject Percentage
1      HD    Math      24.16
2       D    Math      35.91
3       C    Math      24.16
4       P    Math      11.41
5       N    Math       4.03
6 WD-LATE    Math       0.00

and so on: one row for every subject and grade.

Alasdair
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