A graphic is a diagram when correspondences on the plane can be established among all elements of another component. - Jacques Bertin

When the correspondences on the plane can be established between:

Information is the reply to a question.

The plane is the mainstay of all graphic representation. It is so familiar that its properties seem self-evident, but the most familiar things are often the most poorly understood. The plane is homogeneous and has two dimensions. The visual consequences of these properties must be fully explored.

If, in order to obtain a correct and complete answer to a given question, all other things being equal, one construction requires a shorter observation time than another construction, we can say that it is more efficient for this question.