The plots below show three continuous variables.

The line (x=2) appears to separate the points. But it is actually not the case, which we can see by plotting the data in a couple of two-dimensional points.

Why is this happening?

1. Humans are not good at reading pseudo-3D plots.
2. There must be an error in the code.
3. The colors confuse us.
4. Scatterplots should not be used to compare two variables when we have access to 3.