A graphical method for calculating the value of one variable, given the values of other variables. A curve, or straight line, is drawn for each variable, and scales, usually non-linear, are marked on each curve in such a way that the points corresponding to any set of related values of the variables are collinear. Thus a straight line joining the points corresponding to the values of two of the variables will cross the curve corresponding to another variable at the point corresponding to the value of this variable.