Why not just write a little helper function? That way you can call it at the function call.
function combine_cases(x)
combined_cases = map(df.cases) do c
if c in ["A", "B"]
"A or B"
elseif c in ["C", "D"]
"C or D"
else
c
end
end
end
plot(df.x, df.y, group = combined_cases(df.cases))