Incorporating an image as legend for ggplot2 plot

Question

I have an image that I want to use as a custom legend - a universal legend of 3 plots in one figure combined with grid.arrange:

I figure I can load this into R with png or some such package and use grid.arrange to place it next to my graphs. Is it possible to include such images in the package that already contains the functions to make my combined chart, and if so where does one put it and how would you call it from within R code then from it's location within the package. I intend to use this sort of graph with this legend often.

UPDATE:

I've created a folder called 'img' in the root of my Package structure and placed the file inside.

In my R code is the line: legend <- readPNG(system.file("img", "rgblegend.png", package="HybRIDS"), TRUE) However when build the binary, then install it from local zip file, I go to plot and:

Error in readPNG(system.file("img", "rgblegend.png", package = "HybRIDS"), : unable to open If I check my library there is no folder 'img'

EDIT:

I've checked a source version of my package - it includes the 'img' folder, however installing from source also fails to put the folder into my directory. So I guess my question now is, why does making a binary package not include the img folder, and why does the source package include it but not install it to my library - what do I do to correct this?

Answer 1

agstudy's answer describes how to reference a file in an installed package without having to know where the package was installed. The other part of the problem is getting the image file into the installed package.

From the "Package subdirectories" second of Writing R Extensions, the section of interest is about the inst subdirectory:

The contents of the inst subdirectory will be copied recursively to the installation directory. Subdirectories of inst should not interfere with those used by R (currently, R, data, demo, exec, libs, man, help, html and Meta, and earlier versions used latex, R-ex). The copying of the inst happens after src is built so its Makefile can create files to be installed.

So for a file to appear in the img directory off the root of the installed package, it must appear in the inst/img directory of the source. So move rgblegend.png from img to inst/img and then

legend <- readPNG(system.file("img", "rgblegend.png", package="HybRIDS"), TRUE)

should work.

Answer 2

If I understand the question is how you can be sure to find the image within the package. You can put the png under a folder img of your package. and call it using something like this :

img <- readPNG(system.file("img", "Zwrch.png", package=package_name))

The complete solution looks to something like this:

library(ggplot2)     
library(grid)        ## to create the image grobe
library(png)         ## to read the png
library(gridExtra)   ## to arrange the plots
bp <- ggplot(PlantGrowth, aes(x=group, y=weight)) + geom_boxplot()
img <- readPNG(system.file("img", "Zwrch.png", package=package_name))
legend <- rasterGrob(image=img)
grid.arrange(bp,legend)