using multiple size scales in a ggplot

Question

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

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") + 
    scale_size_continuous(range=c(4,20)) + 
    geom_segment( data=trans, aes( x=x.from, y=y.from, xend=x.to, yend=y.to, size=amount ),lineend="round",arrow=arrow(),alpha=0.5)

Example image

I'd like to be able to scale the arrows on a different scale to the circles. Ideally, I'd like a legend with both scales on, but I understand this may not be possible (using two scale colour gradients on one ggplot)

Is there a more elegant way to do this than applying arbitrary scaling to the underlying data?

Sorry, not really an answer, but for plotting networks you can look at specialized libraries such as `igraph` or `network`. — juba, Feb 01 '13 at 17:58
I agree that other libraries might be the way to go. I'm just hoping I can do what I need to in ggplot. Currently using alpha on the arrows and size on the circles works OK, but it's a bit of a hack — mo-seph, Feb 04 '13 at 18:04

score 1 · Answer 1 · edited May 23 '17 at 10:32

A nice option is to generate the circumference of your classes as a series of points, adjusting the scale (diameter) according to your data. Then you draw the circles either as paths or polygons.

Follows some example code. The circleFun was shared by @joran in a previous post. Does this work? I think you should tweak the circle scales acording to your real data.

Important note:
Also, from your use of arrow without attaching grid, I assume you have not updated ggplot2. I changed that code to work with my setup, and tried not to include any ggplot2 code that might cause backward compatibility issues.

# Load packages
library(package=ggplot2)  # You should update ggplot2
library(package=plyr)     # To proccess each class separately


# Your data generating code
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") )


# Generate a set of points in a circumference
# Originally posted by @joran in
# https://stackoverflow.com/questions/6862742/draw-a-circle-with-ggplot2
circleFun <- function(center = c(0,0), diameter = 1, npoints = 100){
    r = diameter / 2
    tt <- seq(0,2*pi,length.out = npoints)
    xx <- center[1] + r * cos(tt)
    yy <- center[2] + r * sin(tt)
    return(data.frame(x = xx, y = yy))
}


# Get max and min sizes and min distances to estimate circle scales
min_size <- min(points$size, na.rm=TRUE)
max_size <- max(points$size, na.rm=TRUE)
xs <- apply(X=combn(x=points$x, m=2), MARGIN=2, diff, na.rm=TRUE)
ys <- apply(X=combn(x=points$y, m=2), MARGIN=2, diff, na.rm=TRUE)
min_dist <- min(abs(c(xs, ys)))  # Seems too small
mean_dist <- mean(abs(c(xs, ys)))

# Adjust sizes
points$fit_size <- points$size * (mean_dist/max_size)


# Generate the circles based on the points
circles <- ddply(.data=points, .variables='class',
                 .fun=function(class){
                    with(class,
                    circleFun(center = c(x, y), diameter=fit_size))
                 })
circles <- merge(circles, points[, c('class', 'size', 'fit_size')])


# Plot
ggplot(data=circles, aes(x=x, y=y)) +
    geom_polygon(aes(group=factor(class), fill=size)) + 
    geom_segment(data=trans,
                 aes(x=x.from, y=y.from, xend=x.to, yend=y.to, size=amount),
                 alpha=0.6, lineend="round", arrow=grid::arrow()) +
    coord_equal()

using multiple size scales in a ggplot

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