Multiple histograms in a grid, each with its own y-axis scale

grid_histo.R

library(ggplot2)

                                        # First get the Iris histogram
histo_list <- lapply(names(iris)[1:4],
                     function (colname) qplot(iris[[colname]], geom = "histogram", xlab = colname))

                                        # Add two sets of random normal samples
n <- 100
histo_list[[5]] <- qplot(rnorm(n), geom='histogram', xlab='rand-norm-1')
histo_list[[6]] <- qplot(rnorm(n), geom='histogram', xlab='rand-norm-2')

multiplot <- function(..., plotlist=NULL, file, cols=1, title=NULL, layout=NULL) {
    library(grid)
    
    ## Make a list from the ... arguments and plotlist
    plots <- c(list(...), plotlist)

    numPlots = length(plots)

    ## If layout is NULL, then use 'cols' to determine layout
    if (is.null(layout)) {
        ## Make the panel
        ## ncol: Number of columns of plots
        ## nrow: Number of rows needed, calculated from # of cols
        layout <- matrix(seq(1, cols * ceiling(numPlots/cols)),
                         ncol = cols, nrow = ceiling(numPlots/cols))
    }

    if (numPlots==1) {
        print(plots[[1]])

    } else {
        ## Set up the page
        grid.newpage()
        pos_shift <- 0
        if ( is.null(title) ) {
            pushViewport(viewport(layout = grid.layout(nrow(layout), ncol(layout))))
        } else {
            pushViewport(viewport(
                layout = grid.layout(1 + nrow(layout), ncol(layout),
                                     heights = unit(c(1, rep(4, nrow(layout))), 'null'))))
            grid.text(title, vp = viewport(layout.pos.row = 1, layout.pos.col = 1:ncol(layout)))
            pos_shift <- 1
        }
        
        ## Make each plot, in the correct location
        for (i in 1:numPlots) {
            ## Get the i,j matrix positions of the regions that contain this subplot
            matchidx <- as.data.frame(which(layout == i, arr.ind = TRUE))

            print(plots[[i]], vp = viewport(layout.pos.row = pos_shift + matchidx$row,
                                            layout.pos.col = matchidx$col))
        }
    }
}

multiplot(plotlist = histo_list, cols = 2, title = "a grid plot")