Why two dimensions? Why not a simple bar chart? Maybe there's a reason for blocks, but I can't tell. I can only see a fraction at a time. There was another xk

Why two dimensions? Why not a simple bar chart? Maybe there's a reason for blocks, but I can't tell. I can only see a fraction at a time. There was another xkcd chart like this before and I didn't see a reason for blocks instead of lines then either.