Avoid "overshooting" of the profile items by linearly clipping
the first and last segment to the boundaries of the time-axis.
Sadly, quite a lot of code, because every profile item is
slightly different.
In particular the pressure-segment handling was rewritten.
It now stores the begin and end of each segment to draw
the appropriate text items.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
073059f5ab