ggplot2 Legend Background Width

require(tikzDevice)
require(ggplot2)
require(reshape2)
require(grid)
dataset <- read.table("ggplot2-legend.dat", header=TRUE, sep=",")

ggplot(data=dataset, aes(x=Concept, y=Classification, fill=Concept)) + geom_boxplot() + geom_point() + scale_y_discrete(labels=c("Novice", "", "", "Expert")) + scale_x_discrete(labels=element_blank())+ theme(legend.position=c(0.82,0.8), legend.title=element_blank(), axis.title.x=element_text(vjust=-2), axis.ticks.y=element_blank(), plot.margin=unit(c(0.2,0.15,0.2,0), "cm")) + theme(axis.text.x = element_text(angle=-90, vjust=0.5)) + labs(x="", y="") + coord_flip() + theme(legend.key.width = unit(3, "line"))