It is the vertical axis along which the abscissa are measured.
Often it is the y-axis, but not always. On a distance time chart, for example, you will probably have t and d-axes rather than x and y. On a probability chart, the vertical axis is the probability, not y. In 3-D space, it is the z-axis that is vertical, not y.
A vertical line in the xy coordinate plane would represent the line of an equation such as x = 1 or x = -4.
The z axis in the vertical axis in the Cartesian Coordinate System.
The horizontal number line on a coordinate plane is called the x-axis, and the vertical number line on a coordinate plane is called the y-axis.
a coordinate system
In a vertical line, all the x values will be the same. This means that all vertical lines will have the equation X=?