This grid of dots is topologically equivalent to the London Underground map.
"It occurred to me that it might be possible to tidy up the map by straightening the lines, evening out the distance between stations. The more I thought about it, the more I thought it was worth trying." – Harry Beck
A drawing for those of us who think Mr. Beck just didn't go far enough.
Every station is exactly 1 unit away from four other stations. By necessity,
these aren't often the stations which are actually connected to it, but I have
tried to make all the interconnections as short as possible. Naturally, no
interconnection passes through a false grid point on the way.