Given only an image, I think about the best we can suggest is to use an image editor (either GUI or command-line-driven) to make the change; that's clearly suboptimal, because it's going to be extremely specific to this one instance. But if you include how the image was generated, and ideally also include an image showing the result you're after, then we might be able to suggest a more general approach.

Yes, I am looking for a GUI solution and mentioned Gimp in the post. I linked to the original inspiring post that shows exactly how this image was created, but it doesn't seem like pandas supports this sort of thing easily.

What I'm missing is how I would create "marks" and put a line exactly in between them in something like Gimp.

I haven't come up with a solution and so there is no final product image. I explained it as best I could and it seems it worked for samcarter‭ as he got exactly what I meant.

mcp‭ Have a look at https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.vlines.html

mcp‭ Unrelated to the question, but if you have control over the source, consider using another format than png. If you can export to pdf, svg or similar, you will have much better quality for this type of graphic than if you use a pixelated format like png