This applet uses the Lorentz Transformation of Special Relativity to transform events from one inertial reference frame to another. The graph on the left shows events in an inertial frame at rest with respect to you. The graph on the right shows events in an inertial frame moving at constant speed (adjust with slider bar) with respect to you. You can add events by clicking on either graph.
In addition, you can turn on gridlines to see how they transform. Gridlines for Your Rest Frame show up in gray on both graphs, and gridlines for the Moving Frame show up in white on both graphs.