arranging ggplot2 legend items in a grid

I have a legend with 4 different entries that I would like to arrange in a 2x2 grid instead of horizontally or vertically. I'm extracting the legend for separate use, which is why I need this somewhat unusual arrangement. Any ideas?

The code below generates the legend with all entries vertically in a single column:


dat <- data.frame(x=c(1,2,1,2,1,2,1,2),y=c(1,2,3,4,2,3,4,5),color=factor(c("a","a","b","b","c","c","d","d")))
p = ggplot(dat)
p = p + geom_line(aes(dat$x,dat$y,color=dat$color))
p = p + scale_colour_manual (values=dat$color,name="")

tmp <- ggplot_gtable(ggplot_build(a.gplot))
leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
legend <- tmp$grobs[[leg]]

legend <- g_legend(p)


Try adding guides(colour = guide_legend(nrow = 2)) to your plot.

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