Change a graph of an accepted conference paper (before submitting the camera-ready version)
Unlike with a journal, the short review and revision timeline of an IEEE conference (or other EE/CS-style conference) means that it is unusual for there to be any opportunity for additional review.  As a result, there is somewhat more trust placed in authors for making "reasonable changes" for the camera-ready, and it is typically not just permitted but even expected that authors will improve their papers for the camera-ready version.  If nothing else, they are expected to do their best to address the issues raised by the reviewers; fixing problems that they themselves have found is entirely desirable as well.  

In short: if your correction does not significantly undermine the basis on which your paper was accepted, you should simply make the change, and everyone will be the better for it.