I assume you're wanting the bars to be in the same order as your "proc sort" produced? (and assuming that your data has 1 obsn per each "bu" ... ie, not having gchart summarize several obsns per each "bu")...
If so, then you can use hbar's "ascending" (or descending) option to control the bar order.
If you wanted the bars to be in some other order (not ascending/descending based on the bar height values), then you would need to get a little trickier, and create a variable that has values of the desired order, and then use a user-defined-format such that that order value will 'print' as the desired "bu" value. (but sounds like you won't need to go to this extra level of complexity).