Don't repeat redundant minima or maxima in the profile plot

If we have more than four identical depth readings, the old code would see
those as local maxima and minima and print spurious depth values in the
profile plot.

Yes, in real sample data identical readings won't happen - but in
synthetic data they can and there this looks really bogus.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>