The relative permeability should be 1 as graphite is nonmagnetic. The relative permittivity can vary greatly depending on the quality of the graphite sample. Overall, the imaginary part of the permittivity should be large because graphite is conductive.

Thank you Dr. Hyde for the answer. Yet, graphite seems to be represented as a diamagnetic material when no intercalation compounds are present in between the layers of graphite. So wouldn't it be diamagnetic?

You are correct. What I meant by nonmagnetic was that you won't measure any significant difference in permeability from that of vacuum.

Thank you Milo Hyde , I was in need of assurance about that fact. Because diamagnetics should exhibit a slightly lower value for real(permeability) than 1. I wonder how it is in microwave frequencies.

So small in fact, it's very likely within the margin of error of your measurement.