Centering title in R viewport with multiple graphs ggplot2

Question

I have a viewport with two graphs drawn in a left to right fashion. I'm trying to have a single title appear over both of the graphs centered in the viewport. Is this possible? Sorry, no pic because of my noob status and I can't post the code because of my work environment.

Answer 1

I think you might be interested in the gridExtra package, which provides the grid.arrange() function that fulfills everything you wonder about.

With @Kevin's example, the command would be

grid.arrange(plots[[1]], plots[[2]], ncol=2, 
             main="test main", sub="subtitle test")

Answer 2

I have been using an almost unmodified version of code in this post to the ggplot2 mailing list to put a main title and subtitle above and below a matrix of plots. Credit to Baptiste Auguié.

arrange <- function(..., nrow=NULL, ncol=NULL, as.table=FALSE, 
                    main=NULL, sub=NULL, plot=TRUE) { 
  dots <- list(...) 
  n <- length(dots) 
  if(is.null(nrow) & is.null(ncol)) { nrow = floor(n/2) ; ncol = ceiling(n/nrow)} 
  if(is.null(nrow)) { nrow = ceiling(n/ncol)} 
  if(is.null(ncol)) { ncol = ceiling(n/nrow)} 
  fg <- frameGrob(layout=grid.layout(nrow,ncol)) 
  ii.p <- 1 
  for(ii.row in seq(1, nrow)){ 
    ii.table.row <- ii.row       
    if(as.table) {ii.table.row <- nrow - ii.table.row + 1} 
    for(ii.col in seq(1, ncol)){ 
      ii.table <- ii.p 
      if(ii.p > n) break 
      fg <- placeGrob(fg, ggplotGrob(dots[[ii.table]]), 
                      row=ii.table.row, col=ii.col) 
      ii.p <- ii.p + 1 
    } 
  } 
  if(!is.null(main) | !is.null(sub)){ 
    g <- frameGrob() # large frame to place title(s) and content 
    g <- packGrob(g, fg) 
    if (!is.null(main)) 
      g <- packGrob(g, textGrob(main), side="top") 
    if (!is.null(sub)) 
      g <- packGrob(g, textGrob(sub), side="bottom") 
  } else { 
    g <- fg 
  } 
  if(plot) grid.draw(g) 
  invisible(g) 
} 

library(ggplot2) 
plots <- llply(1:2, function(.x) qplot(1:10,rnorm(10), main=paste("plot",.x))) 
arrange(plots[[1]],plots[[2]], nrow=1, ncol = 2, as.table=TRUE, main="test main",   
        sub="subtitle test")

Produces:

Answer 3

Another Possibility, it's kind of cheating by way of creating a blank plot...

p1 <- ggplot(diamonds, aes(price,depth)) + geom_point()
p2 <- ggplot(diamonds, aes(price,carat)) + geom_point()
p3 <- ggplot(diamonds, aes(x=1,y=1,label="Title")) + geom_text(size=20) + opts(panel.background=theme_blank(), panel.grid.minor=theme_blank(), panel.grid.major=theme_blank(), axis.text.x=theme_blank(), axis.text.y=theme_blank(), axis.ticks=theme_blank(), axis.title.x=theme_blank(), axis.title.y=theme_blank())
vplayout <- function(x, y) viewport(layout.pos.row=x, layout.pos.col=y)
grid.newpage()
pushViewport(viewport(layout=grid.layout(4,4)))
print(p1,vp=vplayout(2:4,1:2))
print(p2,vp=vplayout(2:4,3:4))
print(p3,vp=vplayout(1,1:4))