We all know that the line of regression is chosen so as to minimize the
residuals but this is often overlooked in the explanations given to
students. This apLet is designed to allow the student to propose a line
and then to adjust their line until the quoted sum of squares of residuals
is as small as possible. 

The arrow keys can be used to move the line up and down and to twist it
clockwise or anticlockwise. The amount of movement is adjustable using
the numeric keys. The residuals are shown as vertical lines from the
points to the proposed line. 

The apLet is a little slow, due to the need to reload the
PLOT screen and then to redraw the lines after each key press but gives a
very effective illustration of the role of the residual in finding the
regression line. This is particularly so when the student choses the
"Show true line" option at the end and finds that their line is
the same as the true one. 