title: Annotate on grob
body: <p>I have combined several ggplots with arrangeGrob.
  Now i'm searching for a way to plot a image over the whole plot.
  For example a watermark which lies over the combined plots.</p>
  
  <p>As the result of arrangeGrob isn't a standard plot or a ggplot object i cant do it the usual way.</p>
  
  <p>Is there a possibility?</p>