Remove o2/he from plot_data and fix gas plot text

plot_data.o2 and plot_data.he was wrong for all dives, due to that
cylinderindex was set right first in populate_pressure_information, and
thus those two contained bogus information.
This makes the plot-text use cylinderindex-lookup as everything else.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>