This page lists three phase plane plotters listed in java. They all work in approximately the same way, but some may work better on some computers than others. I suggest starting with the xFunctions resizable window phase plane plotter, since it is the most flexible. The xFunctions plotters may not work on Macs running Netsacpe 4 and earlier. Try using Internet Explorer. If it doesn't work try the fixed size xFunctions plotter or Scott Herod's phase plane plotter.
The resizable xFunctions phase plotter
The plotter (most convenient)
Instructions for using xFunctions
The fixed size xFunctions phase plotter
Fixed plotterInstructions for using xFunctions
Additional phaseplane plotters
A phase plotter by Scott Herod (most compatible with older browsers)