It is just an X Y plot. Now you can see that the data was not really encoded. The key was the method for applying it.
I left the flashlight graphic out of the initial data so it wasn't too big.
In Jr. highschool back in 1982 my best friend and I made them and we traded them. Not the most efficient way of transferring messages, but it allows for graphics too. It was fun solving them to see what the message was. I had intended on making a computer program so I could type in the line number and letters and display the image onscreen, but he didn't have a computer and we eventually got bored with making them.
I had some of these old papers in a shoe box in the closet and I was reminded of them when I was looking for something else. I thought I'd try it out here on CPF