Fix drawing artifact with UEMIS xml data

Only draw the pressure line to the final data point
(duration / end.mbar) if we haven't already drawn samples
past that point (as the UEMIS records pressure data for a
number of additional samples after the actual dive has ended)

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
[ Changed to use 'last actual drawn sample time that had pressure
  data' instead of 'last sample time'  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>