  Vertex based and spectral based GSP sampling has been studied recently. The
literature recognizes that methods in one domain do not have a counterpart in
the other domain. This paper shows that in fact one can develop a unified graph
signal sampling theory with analogous interpretations in both domains just like
sampling in traditional DSP. To achieve it, we introduce a spectral shift $M$
acting in the spectral domain rather than shift $A$ that acts in the vertex
domain. This leads to a GSP theory that starts from the spectral domain, for
example, linear shift invariant (LSI) filtering in the spectral domain is with
polynomials $P(M)$. We then develop GSP vertex and spectral domain dual
versions for each of the four standard sampling steps of subsampling,
decimation, upsampling, and interpolation. We show how GSP sampling reduces to
DSP sampling when the graph is the directed time cycle graph. Simple examples
illustrate the impact of choices that are available in GSP sampling.
