There are multiple ways to layout graphs on one page with R12. I prefer using grid.arrage function in gridExtra package.

The widths and heights options in the grid.arrage function will help you to arrange graphs with different size. Using function may help you to pass all figures (in the list) to grid.arrange function.

Here is a short example of it.

1.With same figure size

p <- list()
p[[1]]<- ggplot(mtcars, aes(wt, mpg))+geom_point()
p[[2]] <-  ggplot(ChickWeight, aes(x=Time, y=weight, colour=Diet, group=Chick)) +
  geom_line() +
  ggtitle("Growth curve for individual chicks")

p[[3]] <- ggplot(mtcars, aes(x = factor(cyl), fill = factor(am)))+geom_bar()
p[[4]] <- ggplot(subset(ChickWeight, Time==21), aes(x=weight, fill=Diet)) +
  geom_histogram(colour="black", binwidth=50) +
  facet_grid(Diet ~ .) +
  ggtitle("Final weight, by diet") +
  theme(legend.position="none")"grid.arrange", c(p,ncol=2)) 


2.With different figure size

             ncol=2, nrow=2, widths=c(4, 2), heights=c(3, 4))