Add text out of levelplot panel area

Question

I want to add text out of plot area in levelplot. In the following example, I need the text in somewhere in the pointed location.

library (raster)
library(rasterVis)

f <- system.file("external/test.grd", package="raster")
r <- raster(f)
levelplot(r) 

I tried mtext function with no success. Any suggestions?

mtext("text", side=3, line=0)

Answer 1

tldr;

You can annotate the plot using lower-level grid graphical functions. In this case, do something like:

library(grid)
seekViewport("plot_01.legend.top.vp")
grid.text("Hello", x=0, y=unit(1,"npc") + unit(0.4, "lines"), just=c("left", "bottom"),
          gp=gpar(cex=1.6))

---

rasterVis and other lattice-based packages use the grid graphical system, not the base graphical system of which mtext() is a part.

Here, using grid, is how I'd go about adding text at a position 0.4 lines above the upper-left corner of the viewport (a technical grid term) in which that upper margin plot is printed.

• First, find the name of the relevant viewport.

  library(grid)
  levelplot(r)
  grid.ls(viewport=TRUE, grobs=FALSE)  ## Prints out a listing of all viewports in plot
  

  A quick scan of the listing returned by grid.ls() turns up a viewport named plot_01.legend.top.vp, which looks like a promising candidate. If you'd like to check whether it's the correct one, you can plot a rectangle around it with something like the following (which uses the full path to the viewport):

  grid.rect(vp = "plot_01.toplevel.vp::plot_01.legend.top.vp",
            gp = gpar(col = "red"))
  

• Then, using grid's terrifically flexible coordinate system, place the desired text just above the upper left corner of that viewport.

  ll <- seekViewport("plot_01.legend.top.vp")
  grid.text("Hello", x = 0, y = unit(1,"npc") + unit(0.4, "lines"), 
            just = c("left", "bottom"),
            gp = gpar(cex=1.6))
  upViewport(ll)  ## steps back up to viewport from which seekViewport was called