Returns graph number
ifrom the Graph Atlas.
For more information, see
The index of the graph from the atlas to get. The graph at index 0 is assumed to be the null graph.
A list of
Graphobjects, the one at index i corresponding to the graph i in the Graph Atlas.
The time required by this function increases linearly with the argument
i, since it reads a large file sequentially in order to generate the graph .
Ronald C. Read and Robin J. Wilson, An Atlas of Graphs. Oxford University Press, 1998.