layout and layout.drl for other layout
algorithms, plot.igraph and tkplot on how to
plot graphs and star on how to create ring graphs.
Other graph layouts: add_layout_,
  component_wise,
  layout_as_bipartite,
  layout_as_tree,
  layout_in_circle,
  layout_nicely,
  layout_on_grid,
  layout_on_sphere,
  layout_randomly,
  layout_with_dh,
  layout_with_fr,
  layout_with_gem,
  layout_with_graphopt,
  layout_with_kk,
  layout_with_lgl,
  layout_with_mds,
  layout_with_sugiyama,
  layout_, merge_coords,
  norm_coords, normalize