The whole reason people don't like JavaScript weighing down their pages is because they take longer to load. Loading three large images for every update can't be better.
I'd like to point out that this solution is inaccessible to users with impaired vision. That's probably okay for you (because only you are interested in the data), but this makes the solution insufficient for any public usage.
I can assure you the difference in load times between this and using a battle-tested library like D3.js will be imperceptible to the user. You could even throw a thin React app around it, and it would still be blazing fast to the eye.
I am not sure I follow. It's 3 images, so you manually refresh every 5 minutes (maybe that can be automated with meta-refresh so avoiding JS, although I don't see that much advantage over setTimeout?)