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Regarding my comments about the excitation setup and the dip in reflection:
What I meant by excitation setup is the structure (multilayer array with graphene on top) and the plane wave source incident from below, which is used to excite the surface plasmon in graphene.
In the original example, the setup was designed to detect the surface plasmon excitation in graphene, signaled by a dip in the reflection spectrum; in other words, the dip in the reflection was a signature of the excitation of the surface plasmon. Note that in the original example all the materials are dielectrics, no metal is present in the array; therefore, any significant loss comes from graphene, and that is precisely what leads to the dip in the reflection. In your case, you have gold so that makes the configuration quite different.
Also, the analytical results from stackrt from 0 to 90 degrees do not show any particular feature that can be easily associated with a surface plasmon in graphene:
Therefore, your structure behaves quite different from the original example.