ggplot2 for several figure on one page

require(ggplot2)
require(grid)
#####
a <- ggplot(mtcars, aes(mpg, wt, colour = factor(cyl))) + geom_point()
b <- ggplot(diamonds, aes(carat, depth, colour = color)) + geom_point()
c <- ggplot(diamonds, aes(carat, depth, colour = color)) + geom_point() + 
  facet_grid(.~color,scale = "free") 
a

b

c

grid.newpage() 
pushViewport(viewport(layout = grid.layout(2,2))) 

vplayout <- function(x,y){
  viewport(layout.pos.row = x, layout.pos.col = y)
}
print(c, vp = vplayout(1,1:2))   
print(b, vp = vplayout(2,1))   

print(a, vp = vplayout(2,2))   

#dev.off()