Arranging arrows between points nicely in ggplot2

Question

(note - this is the same piece of work as using multiple size scales in a ggplot , but I'm asking a different question)

I'm trying to construct a plot which shows transitions from one class to another. I want to have circles representing each class, and arrows from one class to another representing transitions.

I'm using geom_segment with arrow() to draw the arrows. Is there any way to:

• make the arrows stop before they reach the circles
• adjust the position so that if there is an arrow in both directions, they are "dodged" rather than overlapping.

I couldn't get position="dodge" to do anything useful here.

As an example:

library(ggplot2)
points <- data.frame( x=runif(10), y=runif(10),class=1:10, size=runif(10,min=1000,max=100000) )
trans <- data.frame( from=rep(1:10,times=10), to=rep(1:10,each=10), amount=runif(100)^3 )
trans <- merge( trans, points, by.x="from", by.y="class" )
trans <- merge( trans, points, by.x="to", by.y="class", suffixes=c(".to",".from") )
ggplot( points, aes( x=x, y=y ) ) + geom_point(aes(size=size),color="red",shape=1) + 
    scale_size_continuous(range=c(4,20)) +
    geom_segment( data=trans[trans$amount>0.6,], aes( x=x.from, y=y.from, xend=x.to, yend=y.to ),lineend="round",arrow=arrow(),alpha=0.5, size=0.3)

Answer 1

I thought since nobody has given a solution i would provide an example of package more aimed a this sort of problem:

vecs  <- data.frame(vecs =1:6,size=sample(1:100,6))
edges <- data.frame(from=sample(1:6,9,replace=TRUE), to=sample(1:6,9,replace=TRUE))

library(igraph)

g      <- graph.data.frame(edges, vertices = vecs, directed = TRUE)
coords <- cbind(sample(1:20,6), sample(1:20,6))


plot(g, vertex.size=V(g)$size,vertex.color="white",layout=coords,axes=TRUE)

This will at least solve your arrows before the circle issue and also when there are reciprocal arrows it will adjusts them with the curved lines as in 2<->5 :

(arrrow sizes, line widths, colours etc can of course be modified)

Answer 2

I've put together a simple extension of geom_segment, which allows specification of

• shortening at the start and end of the lines
• an amount to offset lines which share a reversed source and destination

It's up on pastebin here: geom_segment_plus .

I used code along the lines of this:

ggplot( points, aes( x=x, y=y ) ) + geom_point(aes(size=size),color="red",shape=1) +
    scale_size_continuous(range=c(4,20)) + 
    geom_segment_plus( data=trans[trans$amount>0.3,], 
        aes( x=x.from, y=y.from, xend=x.to, yend=y.to ),
        lineend="round",arrow=arrow(length=unit(0.15, "inches")),
        alpha=0.5, size=1.3, 
        offset=0.01, shorten.start=0.03, shorten.end=0.03)

It's definitely not perfect, but it works - you can see a double arrow going to the bottom left point here.

offset, shorten.start and shorten.end are the aes elements added. They can be set to data points, but I haven't figured out how to scale them properly.