In your case (high-level senior management) I don't think the graph type or visual is nearly as important as providing a basic, clearly (and shortly) worded statements of what is being observed - as well as at least what order of magnitude numbers represent "suboptimal or worse". Even a descending line where "smaller is better" could represent a HORRID situation, whereas an increasing chart could be awesome because it is two orders of magnitude away from a pain-point threshold.