This simulation calculates the Keplerian orbit of a planet around a star like our Sun. You can adjust the mass of the planet, the eccentricity of the orbit, and the semi major axis of the orbit.
The first graph shows the positions of the planet (in blue) and the Sun (in red) in the horizontal direction in Astronomical Units (1 Au = 149,597,871 km) as a function of time.
The second graph shows the velocity towards the bottom of the page for the planet (in blue) and the Sun (in red) in km/s as a function of time. Note how these graphs change as you change the eccentricty of the planet and the planet's mass.
The third graph shows just the velocity of the Sun towards the bottom of the page. If you were an observer looking from the bottom of the page up to the Sun, you would be able to measure this radial velocity of the Sun. Astronomers can measure this velocity and use it to infer the existence of unseen plants.