Use the buttons on the right to move the graph or the ones in the middle to alter the scale. The buttons on the left can be used to alter the value of the parameter a. The two "inc" buttons alter the rate at which you can vary the parameter.
BS/JOC/EFR July 1996
All credit for this applet goes to the guys at http://www-history.mcs.st-and.ac.uk/~history/Java/ .