The graph is a convenient way to represent information indicating positions of a variable at various intervals of time. While drawing a graph we first mark points and then join them using the freehand curve in the Cartesian plane.
A Cartesian plane is a 2-dimensional plane having x-coordinates and y-coordinates (or z). The point (0,0) is the origin and is generally taken as a reference point from where all the measurements are done.
Consider a point (a,b) to be marked in a Cartesian plane, thus the point is a unit away from the origin along x-axis (Say point M). From M now move b units parallel to the y-axis. This point finally is the required (a,b) in a Cartesian plane.
For drawing a graph all the points are marked in Cartesian plane following with joining them to form linear graph. Check the detailed solutions for the Introduction to graphs chapter of class 8 maths by RD Sharma which can be asked in your examination.