`Z` axis - depth

The `z` axis in maths is mainly thought of as depth.

In all situations however, the `z` axis is always 90° (perpendicular) to the `x` axis and `y` axis.

A Z axis is an other axis which gives a graph a depth measurement, it is always drawn 90 degrees perpendicular to the other axis

A deep sleep (depth) is lots of Z's (Z-Depth)

Example 1

Z axis drawn in a graph, this axis is not pointing up, its pointing away from the viewer

Example 2

Adding a grid between the x and Z axis shows the floor of the graph

Example 3