require(pixmap)
require(grid)

x <- read.pnm('cruise.pnm')

pdf('output.pdf', width=6, height=4, version="1.4")

par(mar=c(0,0,0,0))

plot(x) # base graphics

y <- c(6, 6.5, 7, 8, 8.5, 8.2, 10, 9.6, 9.7, 9)
# some data like in the picture you gave


# now the grid stuff

pushViewport(viewport(xscale=c(0,10), 
                      yscale=c(0,10)))

grid.rect(x=0:9, 
          y=0, 
          width=1, 
          height=y, 
          default.units="native", 
          gp=gpar(fill="white", alpha=0.7, col="gray", lwd=2),
          just=c("left","bottom"))

grid.rect(x=0:9, 
          y=y, 
          width=1, 
          height= unit(1, "npc") - unit(y, "native") , 
          default.units="native" , 
          gp=gpar(fill="white", col="white"), just=c("left","bottom"))

grid.lines(x=c(0,10), y=c(5, 5), default.units="native", gp=gpar(lwd=2, col="white", lty="dotted"))
  
grid.lines(x=c(0,10), y=c(10, 10), default.units="native", gp=gpar(lwd=2, col="gray", lty="dotted"))

popViewport()


dev.off()