I had information on these rather buried, and did not have a single example
of how to compute them. Now the first part of the chapter covers this much
more thoroughly.
Also added a section on plotting exponentials. Not sure I want to retain
it; it is a bit 'light'.
My charts were mixing position vs time, which was pretty confusing.
I changed it to position vs velocity, and demonstrated how multipying
the covariances lead to a much better result.
