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%MYLOWESS Lowess smoothing, preserving x values
% YS=MYLOWESS(XY,XS) returns the smoothed version of the x/y data in the
% two-column matrix XY, but evaluates the smooth at XS and returns the
% smoothed values in YS. Any values outside the range of XY are taken to
% be equal to the closest values.
if nargin<3 || isempty(span)
span = .3;
% Sort and get smoothed version of xy data
xy = sortrows(xy);
x1 = xy(:,1);
y1 = xy(:,2);
ys1 = smooth(x1,y1,span,'loess');
% Remove repeats so we can interpolate
t = diff(x1)==0;
x1(t)=; ys1(t) = ;
% Interpolate to evaluate this at the xs values
ys = interp1(x1,ys1,xs,'linear',NaN);
% Some of the original points may have x values outside the range of the
% resampled data. Those are now NaN because we could not interpolate them.
% Replace NaN by the closest smoothed value. This amounts to extending the
% smooth curve using a horizontal line.
ys(xs<x1(1)) = ys1(1);
ys(xs>x1(end)) = ys1(end);