Re: [igraph] plotting graphs with edges of various length

[Tamás Nepusz]

> I thought that the commands
> gm<-set.edge.attribute(gm, "weight", index=E(gm), value=weights)
> gm<-set.edge.attribute(gm, "length", index=E(gm), value=weights)
> ensure that plot(gm) yields the edge lengths corresponding to the values
> of vector weights.
No, it doesn’t. *Some* layout algorithms *might* take into account the weights 
of the edges to *some* extent (i.e. they would *try* to ensure that edges with 
larger weights end up being shorter than edges with smaller weights), but there 
is no layout algorithm that would guarantee that the lengths you specify are 
satisfied exactly. The main reason is that most of the length combinations are 
impossible to satisfy in the 2D plane; for instance, it is impossible to plot a 
full graph of size 4 if we want every single edge to be of length 1 (and there 
are infinitely many examples for this).

As far as I know, the Fruchterman-Reingold layout algorithm (see 
layout.fruchterman.reingold) is the only one in igraph that supports weights; 
see http://igraph.sourceforge.net/doc/R/layout.html . But even for the 
Fruchterman-Reingold algorithm, the only thing that the weights do is that they 
make the attraction forces between the endpoints of the edges proportional to 
the weight of the edge.

— 
T.