combine ggplot2 plots

combinePlots.R

library(gridExtra)

getLegend <- function(tp.gplot, position="bottom", box="horizontal"){
    tmp <- ggplot_gtable(ggplot_build(tp.gplot + theme(legend.position = position, legend.box = box)))
    leg <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
    legend <- tmp$grobs[[leg]]
    return(legend)
}

#then extract height or width of legend
legend <- getLegend(p1)
lheight <- sum(legend$height)

#arrange with grid
p <- grid.arrange(arrangeGrob(p1+theme(legend.position="none"),
                         p2+theme(legend.position="none"),
                         p3+theme(legend.position="none"),
                         p4+theme(legend.position="none"),
                         main=bpl$primer),
                         legend,
                         heights=unit.c(unit(1, "npc") - lheight, lheight),
                         nrow=2)