library(ggplot2)
library(gtable)
a = ggplot(data=diamonds, aes(x=price)) + geom_histogram()
b = ggplot(data=iris, aes(x=Sepal.Length, y=Sepal.Width)) + geom_point()
agg = ggplot_gtable(ggplot_build(a))
bgg = ggplot_gtable(ggplot_build(b))

g = gtable(width=unit(c(0.3, 0.5), "null"), height = unit(1, "null"))
g = gtable_add_grob(g, agg, 1,1)
g = gtable_add_grob(g, agg, 1,2)

plot(g)