regardless of the Units preference. This patch checks the preference
and displays it in the appropriate units.
Unlike some of the other cases, I'm checking the value of the Units
setting each time the plot/panel is displayed.
The patch also pulls the 0.62137119 magic constant into a #define
MILES_PER_KM. This constant (and it's inverse, KMS_PER_MILE), occurs
in several files, and (IMHO) really deserves to be pulled into a
separate header, but again, that's work for another day.
--jtc