title: Gnuplot multiplot with one colorbox
body: <p>I have a layout of 3,1 plots using multiplot. All of the three plots have the same scales, so I just want to show one colorbox, on the right margin. However, once I unset the colorbox for the first two plot, the size of the three plot are different.
  Can anybody tell me how to implement this with the three plots have the same size?</p>