You can live between 0 and ~1XX years and you can travel between 0 and x km where the largest case for x is probably the people on the ISS.

So rectangle might make sense but obviously there is no chance for someone that dies as a infant to have traveled as far as an adult. Also the people on the ISS probably wont be the oldest people alive. So the edges would have to be cut off diagonally of sorts.

Or is this more like a "This is the scale of things that humans can observe" type of thing.

So we can see close stuff and really far stuff like stars and we can practically observe from milliseconds up to a century.