ggplot2 - printing plot balloons memory

Question

Is it expected that printing a large-ish ggplot to PDF will cause the RSession memory to balloon? I have a ggplot2 object that is around 72 megabytes. My RSession grows to over 2 gig when printing to PDF. Is this expected? Are there ways to optimize performance? I find that the resulting PDFs are huge ~25meg and I have to use an external program to shrink them down (50kb with no visual loss!). Is there a way to print to PDF with lower quality graphics? Or perhaps some parameter to print or ggplot that I haven't considered?

Answer 1

For large data sets, I find it helpful to pre-process the data before putting together the ggplot (even if ggplot offers the same calculations).

ggplot has to be very general: it cannot predict what stat or geom you want to add later on, so it is very difficult to optimize things there (the split-apply-combine strategy can lead to exploding intermediat memory requirements). OTOH, you know what you want and can pre-calculate accordingly.

The large pdf indicates that you either have a lot of overplotting or you produce objects that are too small to be seen. In both cases, you could gain a lot by applying appropriate summary statistics (e.g. hexbin or boxplot instead of scatterplot).

I think we cannot tell you more without details of what you are doing. So please create a minimal example and/or upload the compressed plot you are producing.

Answer 2

Addressing the second part of your question, R makes no attempt to optimize PDFs. If you are overplotting a lot of points, this results in some ridiculous behavior. You can use qpdf to post-process the PDF.

Addressing the first question anecdotally, it does seem that plots on medium-sized datasets take up a lot of memory, but that is merely my experience. Others may have more opinions as to why or more facts as to whether this is so.