Can you use a value for coloring without showing in tooltip?

[SteveBlackUK]

I'd like to use a value for coloring but not show in my tooltip. The example is a credit rating - e.g., AAA AA A BBB, etc - I want to color based on that, but really I'd have to create numerical values for each bucket. I can do that, but I don't then want the tooltip to show some numerical value that isn't needed when the credit rating itself is what I'll show. I can get them both to show currently, but I'd like to be able to hide an item that is used for coloring. Possible? Thanks!