suggestion from teacher using this simulation.

I suggest a few things:
1) can we have a button for students to reset the duration of orbit, t? So that they can reset t easily for the different planets.
[color=blue]
will add a button for t=0;[/color]

2) under the display option, can we have an option for 'Planet-Sun line'? So that they can 'see' the radius of orbit for planet.

[color=blue]good idea[/color]

3) the model shows that planets Mercury, Saturn and Pluto do not have circular orbit around the Sun (centre of circle does not seem to be at centre of Sun). Is this accurate?

[color=blue]real orbits are not circular and one of the focus is the Sun. the quick answer is it is accurate simulation. :)[/color]
http://www.astro-tom.com/technical_data/elliptical_orbits.htm
Kepler's First Law:
I. The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.[img]http://www.astro-tom.com/images/kepler1.gif[/img]
Kepler's First Law is illustrated in the image shown above. The Sun is not at the center of the ellipse, but is instead at one focus (generally there is nothing at the other focus of the ellipse). The planet then follows the ellipse in its orbit, which means that the Earth-Sun distance is constantly changing as the planet goes around its orbit. For purpose of illustration we have shown the orbit as rather eccentric; remember that the actual orbits are much less eccentric than this.