Why is the graph inside Graham Scan always planar

Question

One of the ways to prove that Graham Scan constructs convex hull in linear time is using planarity of the graph obtained by running the algorithm. This graph is always planar, so according to Euler's formula the number of edges can never be more than $3*n$ (where $n$ is the number of vertices). My question is, how do we know that the edges examined by Graham Scan never intersect (hence always form a planar graph)? Here is the screenshot of this being discussed in the course "Computational Geometry" from Tsinghua University (from EdX.org). The lecturer just mentioned it being true, but did not give the proof.