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The GRAPH data structure used behind Facebook.
for more detail see
1. Get user's permission for the app.
2.Facebook will add "code" get variable on the url declared on the redirect_uri.
3.Using the "code" variable, generate access_token using this link,
4.Graph API will generate the access_token just once
the five graph is line graph,bar graph,pictograph,pie graph and coin graph
bar graph, double bar graph, line graph, and picto graph
its a graph