BestFit: draw points at random around a line.
Do a best fit on the curve showing the squares of the residuals.

Initially the line is done by eye dragging the red and green dots
and when either is released the squares drawn. The measure
shows the total area for a fit going up or down accordingly.
Press on the fit button and the best line is fitted mathematically.            
The idea being that students will hopefully notice the lower the
area, measure, the better the fit looks.