A
two-dimensional graph is a set of points in
two-dimensional space. If the points are
real and if
Cartesian coordinates are used, each
axis depicts the potential values of a particular
real variable. Often the variable on the horizontal axis is called
x and the one on the vertical axis is called
y, in which case the horizontal and vertical axes are sometimes called the
x axis and
y axis respectively. With real variables on the axes, each point in the
graph depicts the values of two real variables.