The old code used the maximum / minimum values of nine-minute
intervals to indicate maximum / minimum depths. This does not
work well when zooming, since the labels will get sparse.
Instead implement a primitive peak finding algorithm, that
searches for the deepest peak in the whole plot and then
repeats the procedure for the right and left sides, leaving
out a certain distance to the origninal peak. This is repeated
until there are no more peaks found.
Only peaks of a certain prominence are considered, which
conveniently gives us the valleys.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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