If you wish, make the entire Plot cell TraditionalForm; that will work OK -- with a reference to separately defined function f (whether that f was defined in an Input cell that had TraditionalForm or StandardForm) or with the function expression directly embedded as the first argument.
Or, if you wish, make just the definition of your function in TraditionalForm. Then use a StandardForm Plot expression.
--- Murray Eisenberg firstname.lastname@example.org Mathematics & Statistics Dept. Lederle Graduate Research Tower phone 413 549-1020 (H) University of Massachusetts 413 55-2838 (W) 710 North Pleasant Street fax 413 545-1801 Amherst, MA 01003-9305