Re: [igraph] Issues trying to get tree using layout.reingold.tilford

[Tamás Nepusz]

> I'm trying to use R igraph to get x,y coordinates to plot vertices in another 
> application.  The data should show a simple organisation chart, hence the 
> desire for a top-down tree.
> [...]
> Looking at l I get something similar to:
> 
>               [,1]          [,2]
> [1,]   0.000000e+00 2.116259e+214
> [2,]   0.000000e+00 2.116259e+214
> [3,]  3.239144e-319 2.116259e+214
> [4,]   0.000000e+00 2.116259e+214
> [5,]  8.156530e-320 2.116259e+214
> [6,]  6.147313e+257  0.000000e+00
> 
> (top lines only) which isn't what I'm expecting.
Seems like this is an issue which arises only in the 64-bit version of R -- at 
least on my machine. Using the layout algorithm from either the R interface on 
a 32-bit R version, or from the Python interface seems to work, although the 
layout does not make sense because layout.reingold.tilford assumes that the 
tree is oriented _downwards_, i.e. it will follow the outbound edges only. So, 
reversing the direction of all of your edges also resolves the issue:

> gdf <- graph(as.vector(t(get.edgelist(gdf, names=F)[,c(2,1)])))
> l <- layout.reingold.tilford(gdf)
> l
      [,1] [,2]
 [1,]  0.0    3
 [2,] -6.7    2
 [3,] -3.2    2
 [4,]  0.3    2
 [5,]  3.3    2
 [6,]  6.3    2
 [7,] -8.2    1
 [8,] -7.2    1
 [9,] -6.2    1
[10,] -5.2    1
[11,] -4.2    1
[12,] -3.2    1
[13,] -2.2    1
[14,] -2.7    0
[15,] -1.7    0
[16,]  0.3    1
[17,] -0.7    0
[18,]  0.3    0
[19,]  1.3    0
[20,]  1.3    1
[21,]  2.3    1
[22,]  3.3    1
[23,]  4.3    1
[24,]  5.3    1
[25,]  6.3    1

I will file a bug for this in our issue tracker. Thanks for the bug report!

-- 
Tamas