Color tank pressure plot based on relative sac

Color tank pressure plot based on relative sac

Linus suggested that instead of using absolute SAC values to base the
color on (which forced us to pre-define which SAC rates are green and
which are red) we should color the tank pressure plot relative to the avg
SAC rate of that dive - which I think makes the coloring much more useful
to spot when on your dive you were doing well and when you were not.

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