I am looking for a way to modify font types in ggplot. At the moment I would be happy enough to simply change fonts to the \'courier\' font family, but ultimately my goal is
You can set the font of the labels produced by geom_text with grid.gedit:
geom_text
grid.gedit
grid.gedit("GRID.text",gp=gpar(fontfamily="mono"))
Call this after you have produced your original plot.