First get the min and max values, then determine the threshold as some fraction from the center and clip:
maxy = max(y)
miny = min(y)
middley = (miny + maxy)/2;
% Determine top point as, say 90% ofthe way from the middle line to the max
fraction = 0.9;
upperThreshold = middley + fraction * (maxy - middley);
lowerThreshold = middley - fraction * (middley - miny);
% Now clip
indexesTooHigh = y > upperThreshold;
y(indexesTooHigh) = upperThreshold;
indexesTooLow = y < lowerThreshold;
y(indexesTooLow) = lowerThreshold;
