Add image (png file) to header of pdf file created with R - r

Question

I am trying to add an .png image (logo) to the header of my pdf report of graphs created with ggplot and printed to pdf.
I found the following example how to add a image to a ggplot plot. But, I am looking to add the .png image to he header of the pdf which is outside the ggplot area.
#-------------------------------------------------------------------------------
# Example png file
#-------------------------------------------------------------------------------
library(reshape2)
library(png)
mypngfile = download.file('http://api.altmetric.com/donut/502878_64x64.png',
destfile = 'mypng.png', mode = 'wb')
mypng = readPNG('mypng.png')
#-------------------------------------------------------------------------------
# create example plot using mtcars data frame from ggplot
#-------------------------------------------------------------------------------
library(ggplot2)
p.example = qplot(mpg, wt, data = mtcars) +
annotation_raster(mypng, ymin = 4.5, ymax= 5, xmin = 30, xmax = 35)
#-------------------------------------------------------------------------------
# print to pdf file with footnote
#-------------------------------------------------------------------------------
fname = "C:/temp/my report.pdf"
pdf(fname, 10.75, 6.5, onefile=TRUE, paper="a4r")
print(p.example)
dev.off()
...which produces a pdf that looks like this:
But, I would like the image to show up outside the ggplot area. Or more specifically, I want the image to show up in the report header (in upper left) like the following example:
I found the following function that can be used to create a text footnote but wasn't sure how to modify it to insert a .png image.
makeFootnote <- function(footnoteText= format(Sys.time(), "%d %b %Y"),
size= .4, color= grey(.5))
{
require(grid)
pushViewport(viewport())
grid.text(label= footnoteText ,
x = unit(1,"npc") - unit(12, "mm"),
y = unit(0.1, "mm"),
just=c("right", "bottom"),
gp=gpar(cex= size, col=color))
popViewport()
}
Any assistance would be greatly appreciated.

Answer 1

here's a suggestion,
library(ggplot2)
p.example = qplot(mpg, wt, data = mtcars)
library(grid)
library(gtable)
ann <- rasterGrob(mypng, width=unit(1,"cm"), x = unit(0.5,"cm"))
g <- ggplotGrob(p.example)
g <- gtable_add_rows(g, grobHeight(ann), 0)
g <- gtable_add_grob(g, ann, t=1, l=4)
grid.newpage()
grid.draw(g)