It is possible to plot a function of one or two variables in a 3D plot region.

This topic compares the plots of such functions.

Define and plot a one variable function f1(x):

The default 3D plot of f1(x) is on the left, and it shows the trace of a line. The plot on the right is the same plot but rotated to verify that the trace is indeed a line.

Plotting the one variable function f1(x) in a 3D region is equivalent to plotting a two variable function f2(u,v) where you only specify one variable, as shown below:

Define a two variable function f3(x,y) in terms of one variable:

The default 3D plot of f3(x,y) is on the left, and it shows a plane. The plot on the right is the same plot but rotated to verify that the trace is indeed a plane.

Plotting the one variable function f3(x,y) in a 3D region is equivalent to plotting a two variable function f4(u,v) where you specify two variables, as shown below: