Grid of multiple ggplot2 plots which have been made in a for loop

Question

as a new ggplot2 user, I am a bit lost with the amount of possibilities, and struggle to find on the net a simple answer to what I consider a simple problem.

I would like to display multiple plots from ggplot2 on a same sheet, BUT knowing that these plots come from a for loop.

Following example does not compile, it is only to illustrate :

for(i in c(1:n)){                                   
  for(j in c(1:m)){
    ..........  # some data production
    p <- ggplot(df.all) + geom_bar(aes_string(x=class.names[i],fill=var.names[j])
}}

Here, p is overwritten, but I would like to have instead a matrix or a list in which I can put all the p as they are produced, then a simple function like

display_in_a_grid(list_of_ggplot_plots)

But as far as I tried, I was not able to make a list of matrix of plot, neither to find a function that takes only one argument for input.

About things I have had a look at :

"arrangeGrob" from package gridExtra doesn't work because it requires an explicit name for each plot (e.g.: p1,p2,p3,...) like in http://code.google.com/p/gridextra/wiki/arrangeGrob

"facet" method of ggplot2 is not adapted to the organization of my data set (or the contrary :p )

Would you have a simple way to manage this ?

Thank you,

François

Answer 1

I would be inclined to agree with Richie, but if you want to arrange them yourself:

library(gridExtra)
library(ggplot2)
p <- list()
for(i in 1:4){
  p[[i]] <- qplot(1:10,10:1,main=i)
}
do.call(grid.arrange,p)

take a look at the examples at the end of ?arrangeGrob for ways to eliminate the for loop altogether:

plots = lapply(1:5, function(.x) qplot(1:10,rnorm(10),main=paste("plot",.x)))
require(gridExtra)
do.call(grid.arrange,  plots)

Answer 2

This is my solution. Tiny change in the ggplot function with the mapping parameter to aes_string.

library(gridExtra)
library(ggplot2)
p <- list()
for(i in 1:4){
p[[i]] <- ggplot(data=df,aes_string(x=df$x,y=df$y) +geom_bar(aes_string(x=class.names[i],fill=var.names[j])
}
do.call(grid.arrange,p)

Hope this helps!