Where is the z axis on a Cartesian coordinate? I googled it and some graphs show the Z axis where the Y axis is.
The picture above is a good illustration of a 3-dimensional graph. Because we can only draw things in 2-dimensions (normally), we have to sort of use our imaginations and depth tricks with a drawing to create the third axis. In the illustration, the regular x-y graph has been turned sideways and then directed towards the observer, creating the feel that the x-axis is coming out towards you and the y axis rests along the width of the page. The z-axis, then, becomes the axis that runs along the height of the page.