title: How to keep previous plots and windows in Gnuplot?
body: <p>I have some problems of using gnuplot. I just begin with it.</p>
  
  <p>(1)What is the command for keeping the previous plot when I plot new data? Do I have to plot the old data and the new data at the same time?</p>
  
  <p>(2)What is the command for open a new window while keep the old ones? Do I have to set the window's id by using, e.g.,
      set terminal wxt 3
  , before each plot?</p>
  
  <p>Can anyone give me some help or some good references?</p>