f : R2 -> R
The dual in space of a point is a plane.
A point in space is determined as the intersection of three planes. 

A plane in space is detemined by the join of three points

The graph of the function is the collection of points in space determined by the planes 
X=a, Y = b, Z= f(a,b). 
The dual graph of the function (Mapping figure) is the collection of planes in space determined by the points 
X=a, Y=b, Z= f(a,b). 

Problem: Suppose f (x,y) = z = 2x - 3y... a linear function.

The points on the graph all lie on a single plane in space.  Question: Do the planes of the dual graph determine a single point in space? 
Answer: ???

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