Please find attached image, Hope it will help you.
Go to Chart Properties -> General Tab -> Error Message Button.
Apply your logic for Custome Error Message.
Untitled picture.JPG 71.2 K
Thanks Rob, it's bad to know that we don't have an option to change formatting.
Yes, I did try the alternative that you were saying, but unfortunately my server is bit slow in loading the objects, so the underlying text box appears first in all cases and then the actual chart, this is looking bit awkward.