A graph is a picture of one or more functions on some part of a coordinate plane. Our paper usually is of limited size. The coordinate plane is INFINITE. Therefore, we need to pick an area of the coordinate plane that would fit on the piece of paper. That means specifying minimum and maximum values of x and y coordinates. This website has a system for displaying graphs (including a solver here that draws graphs). For every graph, you need to specify its size in pixels (screen units), the x min, x max, y min and y max, as well as the function.
Example: graph( 500, 300, -5, 10, -5, 10, x^2-2x-3 )