TIL about upset plots. Really cool! Venn is already unwieldy at three sets, but four is not really doing the job of communicating the set inclusion clearly. Seven is just a geometric curiosity.
In all these pictures, the empty set is missing (-: . Of course you could argue that they’re also missing in Venn diagrams, but it’s common to just point on the outside.
The three lightgray circles in the examples are the empty set.
One example included the empty set, the other did not. It doesn't always make sense to have it.