How to multiplot a list of graphical objects?

[spoggi]

Hi all,
Using a for loop, I can display (using print function) 4 ggplot graphical objects, and I store them as 4 elements of a list, called obj (see code below).
I now would like to plot these 4 graphical objects as 4 panels in a single page using for example the multiplot function. But then I have the following error message:

Error in tapply(X = X, INDEX = x, FUN = FUN, ...) : 
  arguments must have the same size

Could anyone help?
Thks.

Here is a part of my code:
for (j in 1:4){
  load(filenamej)
...
  obj[[j]]<-ggplot(z[[j]],aes(x=rownames(z[[j]]),y=count,fill=factor(count))) + geom_bar(stat="identity")
#note: rownames have different sizes depending on j
  print(obj[[j]])
}
multiplot(plotlist=obj,cols=2)

[Saurabh Agrawal]

I think what you need to do is take the print out of the for loop:

png(filename='plot.png')

multiplot(plotlist=obj, cols=2)

dev.off()

Regards

Saurabh

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[David Guy]

You can also try package 'gridExtra' that enables placement of multiple graphical objects (grobs) on a grid.

It has much more flexibility than multiplot().

HTH

David

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[spoggi]

Hi Saurabh,

Actually I did inser the print function just to confirm that I can plot each graphical object independently. Which is ok. I also get a proper answer when I type summary(obj):

> summary(obj)
     Length Class Mode
[1,] 9      gg    list
[2,] 9      gg    list
[3,] 9      gg    list
[4,] 9      gg    list

BUT then

> png(filename='plot.png')
> multiplot(plotlist=obj,cols=2)
Error in tapply(X = X, INDEX = x, FUN = FUN, ...) : 
  arguments must have same length

I still have this problem! It may be due to the fact that barplots (i.e. my 4 graphical objects) have not the same number of bars?
Sincerely yours.

[Dennis Murphy]

Problem 1: No reproducible example.
Problem 2: You allude to the function multiplot(). Assuming it comes
from an R package and is not one of your own invention, there are at
least six CRAN packages with a function named multiplot(): coefplot,
blowtorch, phonTools, abf2, optiRum and Rmisc, which i got from

library(sos)
findFn("multiplot")

You have not mentioned where your multiplot() function comes from.

David Guy has pointed out a simple solution to your problem:

library(gridExtra)
do.call(grid.arrange, obj)

Note that ggplots are produced from grid graphics, so if you're using
a multiplot() function from somewhere, you should make sure that it
supports grid graphics objects. The gridExtra package does; I have no
idea about the others.

To answer your last question, it is highly unlikely that unequal
numbers of bars are the problem since the plots are saved in separate
objects. Below is a reproducible example.

# Create four data frames - note the unequal number of categories
ds1 <- data.frame(x = sample(LETTERS[1:5], 100, replace = TRUE))
ds2 <- data.frame(x = sample(LETTERS[1:4], 100, replace = TRUE))
ds3 <- data.frame(x = sample(LETTERS[1:6], 100, replace = TRUE))
ds4 <- data.frame(x = sample(LETTERS[1:5], 100, replace = TRUE))

# Create a simple function to plot a single data frame
plotfun <- function(d)
{
require(ggplot2)
ggplot(d, aes(x = x)) + geom_bar(fill = "darkorange")
}

# Create a list of ggplot objects
L <- lapply(paste0("ds", 1:4), function(z) plotfun(get(z)))

# Use do.call() to invoke grid.arrange() on the list of plots
library(gridExtra)
do.call(grid.arrange, L)


## Another way that avoids do.call() and allows
# grid.arrange() arguments to be specified - plot
# objects are in the global environment rather than a list object

p1 <- ggplot(ds1, aes(x = x)) + geom_bar(fill = "darkorange")
p2 <- p1 %+% ds2 # Same plot structure as p1, using data ds2
p3 <- p1 %+% ds3
p4 <- p1 %+% ds4

grid.arrange(p1, p2, p3, p4, ncol = 2)

Dennis

[spoggi]

Thanks a lot Dennis for this thorough answer. I finally solved my problem.
Thks again to all for your help.