Indicate vertical velocity through color

Indicate vertical velocity through color

So far Linus has hated all of my attempts to visualize vertical velocity
through color. This time I'm trying something dramatically new: there is
no PURPLE involved. Maybe that will convince him of the value.

We simply calculate the vertical velocity for the current plot segment
(last sample point to this sample point - in this version even without
divisions by zero) and assign a label based on the rate of change. These
labels are translated through a predefined table into colors:

Dark green is +/- 5ft/min (stable)
Light green is descents up to 30ft/min and ascents up to 15ft/min
Yellow is descents up to 60ft/min and ascents up to 30ft/min
Orange is descents up to 100ft/min and ascents up to 60ft/min
Red is outside of those ranges - you are most likely in danger

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>