Maybe you can come up with some complex expression to sort it like you need.
But I believe there are more simple ways to achieve this, since your credit ratings should be a defined set of values:
Either load your set of credit ratings with a inline load prior loading your tables with the ratings, thus the load order of the table could be used as sort order:
LOAD * INLINE [
(add more lines as needed. In your chart, use load order as sort order)
Or explicitely set a numerical to each string, using this numerical as sort order:
LOAD dual(Rating,Sortorder) as Rating INLINE [
Hope this helps,