Use of recordPlot() and replayPlot() in Parallel in R to save plot in the same PDF - r

Question

I would like to plot data in parallel using foreach in R but I didn't find any way to get all my plots in the same pdf file. I thought of using recordPlot to save my plots in a list and then print them in a pdf device but it doesn't work.
I have the following error :
Error in replayPlot(x) : loading snapshot from a different session
I tried as well with ggplot but this is to slow with my large dataset.
Here is a piece of code showing my problem :
# Creating a dataframe : df
df=as.data.frame(matrix(nrow=1, ncol=10))
df=apply(df, 2, function(x) runif(100))
# Plotting function
par.plot=function(dat){
plot(dat)
p=recordPlot()
return(p)}
#Applying the function in parallel
library("parallel")
library("foreach")
library("doParallel")
cl <- makeCluster(detectCores())
registerDoParallel(cl, cores = detectCores())
plot.lst = foreach(i = 1:nrow(df)) %dopar% {
par.plot(df[i,])
}
# Trying to get 1st plot
plot.lst[[1]]
Error in replayPlot(x) : loading snapshot from a different session
Replacing %dopar% by %do% is working when I try to get my plots, because they seems to have been generated in the same environment.
I know I can call a pdf device inside the loop to generate a file for each iteration, but I would like to know if there is a way to get one file for all my plots at the output of my function.
Or do you know an easy way to merge my pdf files afterwards ?
Thanks for your help.
Charles

Answer 1

In my opinion your question can be devided into two distinctive parts:
1. Using the replayPlot function in th%dopar% without getting the weird error
2. Somehow getting 1 file at the end
The first question is easy to answer. The reason you get this error is that the R somehow remembers where (in OS level) the plots has been generated. You can get the same effect by using Rstudio server and trying to replay some of the recorded plots after couple of hours of closing the browser tab. In brief, the issue is that R remembers the PID of the process that generated the plot (Don't know why though!):
# generate a plot
plot(iris[, 1:2]
# record the plot
myplot <- recordPlot()
# check the PID
attr(x = myplot, which = "pid")
the good thing is you can overwrite this by assigning your current PID:
attr(x = myplot, which = "pid") <- Sys.getpid()
so you should only change the last line of your code to the following:
pdf(file = "plot.lst.pdf"))
graphics.off()
lapply(plot.lst, function(x){
attr(x = x, which = "pid") <- Sys.getpid()
replayPlot(x)})
graphics.off()
The part above entirely solves your problem, but in case you are interested in merging PDF files, follow this discussion:
Merging existing PDF files using R