[R] proportional symbol map ggplot

[Strategische Analyse CSD Hasselt]

Hello,

we want to plot a proportional symbol map with ggplot. Symbols' area should 
have the same proportions as the scaled variable.
Hereby an example we found on 
http://www.r-bloggers.com/bubble-chart-by-using-ggplot2/ . In this example 
we see the proportions of the symbols' area are different from the 
proportions of the scaled variable:

crime <- 
read.csv("http://datasets.flowingdata.com/crimeRatesByState2008.csv", 
header=TRUE, sep="\t")
p <- ggplot(crime, aes(murder,burglary,size=population, label=state))
p <- p+geom_point(colour="red") +scale_area(to=c(1,20))+geom_text(size=3)

Example:
proportion population Pennsylvania/Tennessee= 2.003
proportion symbols' area Pennsylvania/Tennessee= +/- 2.50

proportion population California/Florida= 2.005
proportion symbols' area California/Florida= +/-2.25

What we would like is that the proportion of the symbols' area is also equal 
to 2.0.

We see the same in the legend:
proportion population 1.6e+07 / 4.0e+06 = 4.0
proportion symbols' area 1.6e+07 / 4.0e+06= +/-5.0


Thanks in advance!

Ann Frederix
Robbie Heremans