Part of the reason, I think, for the explicit "flatness" of the circle in the meme/line is that time is particularly stuck in endless repetition.

If a circle was not flat, or in other words if it was rising or descending, then it could be said that it is a spiral. A spiral could be ascending or descending, in which case you could say history or time is progressing or degressing. But this implies change and movement. The particular pessimism of the line is that there is no change or end-point to history, no growth or degradation, it is not even as meaningful as any of that, time is simply just stuck in flat repetition.