title: Gnuplot script produces picture with black background instead of expected picture
body: <p>This is part of a gnuplot script I use:</p>
  
  <pre><code>set terminal png transparent
  #plot "back.png" binary filetype=png w rgbimage
  filename = "/MyData/MyRadio/ND/Server/GNUPlot/anim/map_".i.".dat"
  plotfile = "/MyData/MyRadio/ND/Server/GNUPlot/anim/graph_".i.".png"
  set output plotfile
  set ylabel "Latitude"
  set xlabel "Longitude"
  set cbrange [0.5e9:3e9]
  set palette defined (0.5 "blue", 1 "green",1.5 "yellow", 2.1 "orange", 3 "red")
  plot 'back.png' binary filetype=png center=(0,0) with rgbimage notitle, \
  filename using  2:3:4 title columnheader(1) with points pt 7 ps 2 palette
  set output
  i=i+1
  if (i &lt;= n) reread
  </code></pre>
  
  <p>However, it produces a <code>.png</code> file with black background instead of a picture:</p>
  
  <p><img src="https://i.stack.imgur.com/eVqWr.png" alt="like this">.</p>