loop for multiple ggplot and arrange them in the rows and columns you wish for

mutiggplot

###########################Header##############################################
# title         : multiggplot.R;
# purpose       : create muti histogram of all varibles in the dataset;
# producer      : prepared by S. Jaisong ([email protected]);
# last update   : in Los Baños, Laguna, PHL, April 2015;
# inputs        : "airquality" data from R;
# outputs       : exaple.pdf;
# remarks 1     : ;
# remarks 2     : ;
###############################End#############################################
#==== Start loading the packages
library(reshape)
library(reshape2) 
library(ggplot2)
library(gridExtra)
#===== End loading the packages =============

#==> what is your dataframe
dat <- airquality # what is your data frame that you want to plot the graphs
#========================================================================
varnames <- names(dat) # creat the list of variables of you data frame
m.dat <- melt(dat)
names(m.dat)


#============== loop for create multiple graphs of each variable in the data frame
i <- 1
out <- NULL # crerate the list for saving the output file from the ggplot

for(i in 1:length(varnames)) {
        temp <- subset(m.dat, variable %in% varnames[i])
        p <- ggplot(temp, aes(x = value)) + 
                geom_histogram(stat = "bin") + ggtitle(paste("Histogram of", varnames[i], sep = " "))
        dev.new()
        print(p) 
        out[[i]] <- p
}

pdf(file = "example.pdf", width = 20, height = 15) # define the name of outout graphs

g <- grid.arrange(out[[1]],
                  out[[2]],
                  out[[3]],
                  out[[4]],
                  nrow = 2 # layout graphs 2 rows
)
dev.off()