The use of x as the vertical axis coordinate for the vertically moving mass and
then plotting it as the abscissa in the plot may be a little confusing for some,
but instructive. The use of a horizontal spring helps this somewhat, but also
might need some explanation.
Sometimes it is difficult to control whether you are taking off mass or whether
you are trying to start/stop motion.
It is unclear what all the variables labeled on the simulation are.