show the displays available to be used in plot functions. dh: Davidson and Harels algorithm, mds:multidimensional scaling of nodes fr: algorithm of Fruchterman and Reingold. igraph::with_fr() kk: Kamada and Kawai igraph::with_kk(). Kamada, T. and Kawai, S.: An Algorithm for Drawing General Undirected Graphs. _Information Processing Letters_, 31/1, 7-15, 1989. drl: force directed algorithm from the DrL toolbox igraph::with_drl() lgl: algorithm from Large Graph Layout. igraph::with_lgl()