You likely have log-scale turned on, which isn't compatible with symbols that drop below 0. To turn this off, right-click, select Scaling, then Log Scale.
Jay, yes, thx, that's what it was. It confused me when the support staff person seemed to concur that a problem existed. I hadn't checked scaling because I never changed it.
Is there any circumstances whereby the scaling will just change on its own?
Not that I've seen. It could have been a number of things that may have caused it.
1) It was accidently clicked either in the right-click menu or Adv. Chart Toolbar.
2) A Style Template that had that setting already set.
3) A corrupt page/layout that turned that feature on.
