538-ify ggplot output: justifying pictureGrob in R grid graphics

I am trying to write a little utility function that will annotate my plots Reuben Fischer-Baum style with a nice little logo and url at the bottom - eg:

I've figured out how to get a postscript file loaded into R; it's justifying the image on the bottom of the plot that's driving me crazy. I can't get the pictureGrob to left justify; at best I can get it to center on the left hand side of my plot, but half of my image gets cut off, like this reproducible example below:

Tinkering with the width setting can sort of approximate what I'm trying to accomplish by limiting the buffer around the pictureGrob, but what I really want, I think, is to left justify the picture itself inside of its viewport OR make the viewport completely snug to the picture. [Paul Murrell does something similar on p. 162 of the grid book, but that seems to depend on a stringWidth utility function that has no partner for pictures(?)]

I've asked a similar question about gridextra tables, but as baptiste comments there, 'Justification is always rather awkward to handle in grid'.

Any help is appreciated; a pointer to any conceptual discussion of what is happening with justification in grid would be spectacular.

Reproducible code (note that grImport requires a ghostscript install):

require(grImport)
require(ggplot2)
require(gridExtra)

#nb: you also need ghostscript installed on your machine.
#I had a hell of a time getting ghostscript to work properly.  
#From the bowels of a R mailing list, I found this helpful tip 
#from Paul Murrell himself:
#https://www.mail-archive.com/r-help@r-project.org/msg203180.html

#YMMV, THIS WORKED ON MY MACHINE:
  #gswin <- shortPathName("C:/Program Files/gs/gs9.15/bin/gswin64c.exe")
  #Sys.setenv(R_GSCMD = gswin)


#get a fun little moon
PostScriptTrace(
  file=file.path(system.file(package = "RGraphics"), "extra", "comic_moon.ps"),
  outfilename="comic_moon.xml")
moon <- readPicture("comic_moon.xml")


annotate_me <- function(
  plot_to_annotate,
  some_picture,
  bg_color="gray30"
) {

  #make a gTree
  foo <- gTree(
    children=gList(
      rectGrob(gp=gpar(fill=bg_color)),
      pictureGrob(
        picture=some_picture,
        x=0, y=0.5,
        #width=?
      )
    )
  )

  final <- arrangeGrob(
    plot_to_annotate, foo,
    nrow=2, heights=c(19,1)
  )

  return(final)
}


annotate_me(
  plot_to_annotate=qplot(mpg, data=mtcars),
  some_picture=moon
)

I believe widthDetails isn't implemented for pictureGrobs, so justification does random things.

Here's one approach: define the width of your object, and place it at x = 0.5 width.

require(grImport)
hourglass <- new("Picture",
                 paths= list(new("PictureFill",
                                 x=c(0, 1, 0, 1),
                                 y=c(0, 0, 1, 1))),
                 summary= new("PictureSummary",
                              numPaths=1,
                              xscale=c(0, 1),
                              yscale=c(0, 1)))

g1 <- pictureGrob(hourglass, use.gc=FALSE,x=unit(1,"line"),
                  width=unit(2,"line"), gp=gpar(fill="red"))
library(ggplot2)
library(gtable)
library(dplyr)

p <- qplot(1,1)
g <- ggplotGrob(p) 

grid.newpage()
g %>%
  gtable_add_rows(unit(2,"lines")) %>%
  gtable_add_grob(rectGrob(gp=gpar(fill="grey")), name="border",
                  t=nrow(g)+1,l=1,r=ncol(g)) %>%
  gtable_add_grob(g1, t=nrow(g)+1,l=1,r=ncol(g)) %>%
  gtable_add_grob(textGrob("text here", x=1, hjust=1, gp=gpar(col="white")),
                  name = "text", t=nrow(g)+1,l=1,r=ncol(g)) %>%
  grid.draw