Extrapolating distance from a known scale (reducing the mesuring error)

measure lenght
I have been using ImageJ to process images, but I am still quite new to it. I have a quite specific need I don’t quite know how to do in a proper, scientific and mathematical way. I am trying to “extrapolate” numerical values from graphs for a review based on a predetermined scale (known distance), as most papers I will be using do not provide numerical values. Let me show an example.

So, I use the scale tool to set the known distance and then I can measure the graphs in a “quite” accurate way, there is no problem with that. But this is done manually. I will be working with a large sample of graphs and to make it more accurate I would like to reduce the “human” element. I don’t mind doing it manually but I am afraid reviewers might say its too arbitrary, so an algorithm or something “automatic” would be better, I think.

I am wondering if there might be a way to measure distances in a more accurate way, or for example setting the scale and then measuring on the axis with a horizontal line, or measuring by color changes? I am not sure if any of these ways make sense hehe Maybe I could repurpose some plugin that already exists for this, but I am not sure which one might be the best suited.