title: How to plot with a png as background?
body: <p>I made a plot with a 3 million points and saved it as PNG. It took a few hours and I would like to avoid re-drawing all the points.  </p>
  
  <p><img src="https://i.stack.imgur.com/hELX9.png" alt="enter image description here"></p>
  
  <p>How can I generate a new plot that has this PNG as a background?</p>