I finally made a final version out of this. It ended up being a web page http://www.sesquibits.com/mars
One link, How far are the planets from Earth? (sesquibits.com) does what the initial goal intended, to continuously update a display of relative distance, speed and acceleration.
The other, bigger page How far is Mars? (sesquibits.com) lets the user search for events like maximum distance and opposition.
Both pages can be retargeted to any planet.
One feature that I like is that you can create a link to the current display For example, How far is Mars? (sesquibits.com) is the opposition of Mars wrt. Earth that happened.
[Perhaps I should change the title when you do capture image so that you can see what it represents in the title.]