The Setup Menu let's you change the horizontal and vertical limits of the graph as well as the spacing on the axes. Not every axes spacing will work. The D3 javascript used to generate the graph can override your choice as it tries to give a nice axes spacing.
You choose which observer is stationary (the regular black and cyan axes) and which is moving (the tilted red and pink axes) and choose the relative velocity β of the moving observer. β can be positive (moving right) or negative (moving left).
As you move the mouse pointer across the graph, the location of the mouse is given in both frames at the bottom left of the graph.
Double click anywhere on the graph and a small blue circle and its coordinates will appear. This will let you label the coordinates of important Events in both frames. The text of the coordinates may be dragged around the graph.
The Add Lines Menu let's you add horizontal and vertical lines in both frames as well as lines represnting the speed of light. You can also add lines representing objects of arbitrary speed in either frame. Be sure to specify the speed first!
The lines may be dragged anywhere around the graph.
If you hold down the SHIFT key while moving the mouse vertically with the left mouse button pressed, the line will change length.
You can also add helpful one-line notes to the screen such as "Rocket sends radio message to earth".
The notes may be dragged anywhere around the graph.
If you hold the right mouse button down while dragging, the note will rotate.
You can clear the last line, note, or event one at a time or all at once.
If you change the graph limits in Setup, the Events, Lines, and Notes will move to the correct position. The Lines may need to be made longer or shorter though.
If you change β in Setup, the Events, Lines, and Notes will all disappear.