The Map is Computed Using a Continuous Version of the Gravity Model
The result is a system of partial differential equations solved by a finite difference iteration to obtain the potential field.
This can be contoured and its gradient computed and drawn on a map.
W. Tobler, 1981,"A Model of Geographic Movement", Geographical Analysis, 13 (1): 1-20
G. Dorigo, & Tobler, W., 1983, “Push Pull Migration Laws”, Annals, AAG, 73(1):1-17.