Nomographs:

A nomograph is devices used to associate the physical quantities in such a way in which the value of an unknown quantity can be determined provide the values of the other related quantities. Nomographs generally include the relationship among three physical quantities. The scales are situated in such a way that, while a straight line is drawn among the values of the known quantities on their respective scales, the line crosses the value of the unknown quantity on its scale.  Below figure is a typical nomograph which relates the average speed, the distance travelled, and the time traveled. It should be remember that, as with any graphical representation, the values denoted are only approximations.

Figure: Typical Nomograph