Adding a watermark to your plots

 library(ggplot2)
 library(grid)

 qplot(1:10, rnorm(10)) +
annotate("text", x = Inf, y = 0, label = "@Amit Kumar Garg 2015", hjust=1.1, vjust=-1.1, col="white", cex=6, fontface = "bold", alpha = 0.8)

 qplot(1:10, rnorm(10)) +
     annotate("text", x = Inf, y = 0, label = "@Amit Kumar Garg 2015",vjust=-1, col="white", cex=6, fontface = "bold", alpha = 0.8,srt = 90)

Adding a plot as watermark on your main plot

 base <- qplot(1:10, 1:10, geom = "blank") + theme_bw()
 g <- ggplotGrob(qplot(1, 1) +
  theme(plot.background = element_rect(colour = "black")))
base +
  annotation_custom(grob = g, xmin = 1, xmax = 10, ymin = 8, ymax = 10)

#Inf,-Inf

Note that the echo = FALSE parameter was added to the code chunk to prevent printing of the R code that generated the plot.

Adding a very nice confidential in the middle of plot with date.

run.date <- format(Sys.Date(), "%m-%d-%Y")
    #png(paste0("Watermark 1 - ", run.date, ".png"))
    plot(rnorm(100))
    text(x = grconvertX(0.5, from = "npc"),  # align to center of plot X axis
       y = grconvertY(0.5, from = "npc"), # align to center of plot Y axis
        labels = "CONFIDENTIAL", # our watermark
        cex = 3, font = 2, # large, bold font - hard to miss
        col = rgb(1, 0, 0, .2), # translucent (0.2 = 20%) red color
        srt = 45) # srt = angle of text: 45 degree angle to X axis
    # Add another watermark in lower (side = 1) right (adj = 1) corner
    watermark <- paste("Data", run.date)
    mtext(watermark, side = 1, line = -1, adj = 1, col = rgb(1, 0, 0, .2), cex = 1.2)

#dev.off() # close device, create image