There were two plot_infos of the same dive: one owned by
ProfileScene and one owned by DivePlotDataModel. The latter
was (or at least should have been) a copy of the former.
Simply always access the plot-info which is owned by
ProfileScene anyway. That seems much less brittle. Why
risk some desyncing?
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>