how to plot the lower or upper boundary lines for scatter data?

2018 1 29
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更新时间：2018 1 30
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Hi
I have a scatter plot. I'd like to plot the lines connecting the points either the upper or lower bound.
How to do it?
Could you please help? Thank you so much.

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Walter Roberson
Walter Roberson 2018-1-29

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You could use boundary() to help figure out which points are on the perimeter. Figuring out which of those points are on the upper or lower branch might take some thinking for irregular point distributions.

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roudan
roudan 2018-1-30
Thanks Walter. It indeed takes some time to figure out how to find the lower bounds points.
Here are my codes and it works for my case
% if trendline is for boundary line
if ~isempty(strfind(trendlinecoverage,'bound')) % find the lowerbound string
k=boundary(xdata_crossplot',ydata_crossplot'); % have to be column vector
boundarypoint=[];
startx=k(1);
maxindex=length(k);
boundarypoint=[boundarypoint startx];
for i=2:maxindex
currentpointindex=k(i);
previouspointindex=k(i-1);
if xdata_crossplot(currentpointindex)>xdata_crossplot(previouspointindex) % we are moving in x+ direction to find the lower boundary point
boundarypoint=[boundarypoint k(i)];
end
end
hold(hrightsubplot,'on');
legendlabel_boundary='lower boundary line'
hp=plot(hrightsubplot,xdata_crossplot(boundarypoint),ydata_crossplot(boundarypoint),'-b','linewidth',2,'DisplayName',legendlabel_boundary);

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