In a graphic, the two main axes can be used to represent two types of data related to the same subject, and the position in the graphic will establish how they are related. For example, you can think of the horizontal axis as representing time in minutes, and the vertical axis to represent a number of things that increase by one per minute, therefor, if they both start at 0, you will have to move up one unit vertically for every unit you move right horizontally.
