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

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.

Answer 1

[Walter Roberson]

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.

[roudan]

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);