This is a modified and simplified version of this Twin Paradox Applet authord by Rob Salgado: 

WHAT YOU CAN DO
BOOSTING
Use the scrollbaror the [LEFT-ARROW] and [RIGHT-ARROW] keysto adjust the boostVelocity(i.e. perform a Galilean or Lorentz transformation).The applet will display the spacetime diagram for an inertialobserver traveling with that velocity.
Note:
-1 < boostVelocity < 1.
DRAGGING
Click-down and drag the mouse to slide the diagram around.
ZOOMING
ZOOM IN and OUT with the applet-buttons "Z" and "z"or the [UP-ARROW] and [DOWN-ARROW] keys.
RESETTING
To reset the display, hit [R].
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