Here are some screenshots of my first graphics editor; it was for the original Lowres NX. I wanted to edit more than 256 characters, and this program was able to use the entire memory to edit them. The program was never finished, though.

I implemented an undo feature capable of undoing multiple changes.
When I discovered that users would have to choose which set of characters they needed for the background editor, I found the task too difficult to implement and stopped the project. I managed to find a way later on.