I have never seen bills come through as graphs. Sounds like there is some corruption in your reports. I would want to see it first. What happens if you go to bills and statement layouts and open a layout? Does it look normal? Are the proper layouts assigned to the clients?

The solution may be to create an empty database and then replace the rptdef and rptgrp files. However, very hesitant to suggest this without viewing the issue and the bigger pictures.