The simulator allows you to vary these parameters, and see how your changes affect the motion of the pendulum. The strength of gravity, and the value of the spring constant can be changed by moving the appropriate sliders in the center panel. You can also vary the strength of each magnet by moving the corresponding slider. Magnetic field strength is indicated by the size of the blue circle around the magnet.
Grab hold of the pendulum with your mouse by holding down the Shift key while hovering over either the 'above' or 'side' view panels. Magnets can be added or removed manually at any time, and a number of pre-set arrangements can be accessed by clicking the links in the center panel.
To see the entire path that the magnet has followed, click the 'Show Trace' button.
The source code for the pendulum is available on GitHub.