ggplot2 multiply graphs on one figure

library(ggplot2)
library(grid)
grid.newpage()
pushViewport(viewport(layout = grid.layout(1,2)))
vplayout = function(x, y) viewport(layout.pos.row = x, layout.pos.col = y)
b<-matrix(1:2,ncol=2,byrow=T)
for (i in 1:2){
  r<-which(b==i,arr.ind=T)
  read.table(paste("JTK.maize",i,".incluster.txt",sep=''),header=F)->data1
  p<-ggplot(data1, aes(V2))+geom_histogram(breaks=seq(0,24,2),col="black",fill="lightgrey")+labs(title=paste("Maize",i,sep=' '),x="LAG")+theme(axis.line = element_line(colour = "black",size = 0.5),panel.background=element_rect(fill='transparent', color='gray'),legend.key=element_rect(fill='transparent', color='transparent'),axis.text=element_text(color="black",size=10))+scale_x_continuous(limits=c(0,24),breaks=seq(0,24,4))
  print(p,vp = vplayout(r[1],r[2]))
}