#' Multiple plot function
#'
#' ggplot objects can be passed in ..., or to plotlist (as a list of ggplot objects)
#' - cols:   Number of columns in layout
#' - layout: A matrix specifying the layout. If present, 'cols' is ignored.
#' If the layout is something like matrix(c(1,2,3,3), nrow=2, byrow=TRUE),
#' then plot 1 will go in the upper left, 2 will go in the upper right, and
#' 3 will go all the way across the bottom.
#' @param plotlist NULL
#' @keywords multiplot
#' @export
#' @examples
#' multiplot()
multiplot <- function(..., plotlist=NULL, file, cols=1, layout=NULL) {
   require(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()
         pushViewport(viewport(layout = grid.layout(nrow(layout), ncol(layout))))

# 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 = matchidx$row,
                        layout.pos.col = matchidx$col))
         }
   }
}


