# How to find X and Y coordinates of maximum gap between curves?

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MP on 10 Aug 2022
Moved: per isakson on 26 Aug 2022
Please see the attached figure. In this figure, I want to find the values of X and Y coordinates where the two curve has maximum separation in Y-direction. In this figure, we can see that maximum gap is somewhere around 0.7 in the x-scale.
To do so, I simply take the subtraction of red data and blue data and obtain the gap value and corresponding X-value.
The gap value is delta Y is 6.7*10^7 and X-value is 0.027451.
However, the gap is not maximum at X = 0.027451. So, is there any way I can find the X-Y values of maximum gap between curves? The data is also attached here with. It may not be same dataset as shown in figure, but a similar data.
I attach fig file for the same data for refrence.
Here is the code to obtain this type of plot.
But how to find maximum gap between curves?
clear; close all
X = flux1.vapa_flx_E
plot(X,rd,'-or');
hold on;
plot(X,bl,'-ob');
set(gca,'YScale','log','YTick',[10^2 10^3 10^4 10^5 10^6 10^7 10^8 10^9 10^10 10^11],'LineWidth',1.5,'FontWeight','bold',...
'FontSize',14, 'YTicklabel',{'10^2','','10^4','','10^6','','10^8','','10^1^0',''});
dif1 = rd-bl;
[val_maxdif1, idx_maxdif1] = max(dif1);
delta_y = val_maxdif1;
x1 = X(idx_maxdif1);
Any help is greatly appriciated.

Karim on 10 Aug 2022
Edited: Karim on 11 Aug 2022
It depends on what you mean with the gap, notice that you plot it on a logarithmic axis. As a result from a graphical point of view the gap seems larger at 0.7 however if you make the axis linear you will see that the gap is bigger at the small X values
% create figure
figure
plot(X,rd,'-or');
hold on;
plot(X,bl,'-ob');
set(gca,'YScale','log','YTick',[10^2 10^3 10^4 10^5 10^6 10^7 10^8 10^9 10^10 10^11],...
'LineWidth',1.5,'FontWeight','bold','FontSize',14,...
'YTicklabel',{'10^2','','10^4','','10^6','','10^8','','10^1^0',''});
grid on
title('Logarithmic Y axis') figure
plot(X,rd,'-or');
hold on;
plot(X,bl,'-ob');
set(gca,'LineWidth',1.5,'FontWeight','bold','FontSize',14);
grid on
title('Linear Y axis') dif1 = rd-bl;
figure
plot(X,dif1)
grid on update after extra comment of the OP
here i made an attempt to identify the 'gap' that the OP looks for
dif2 = (rd-bl)./bl;
figure
plot(X,dif2)
grid on [maxGap, ixd] = max( dif2 );
maxGap_X = X(ixd);
disp(['max gap is found at X = ', num2str(maxGap_X)])
max gap is found at X = 0.692
Karim on 12 Aug 2022
Thank you, I had some issues a while back because we are limited to the amount of files we can upload to the answers per day. Hence I looked a bit for an workaround.
For completness, I also use the matfile command to see which variables are inside the mat file.
matfile('rd.mat')
ans =
matlab.io.MatFile Properties: Properties.Source: '/users/mss.system.KkVnZh/rd.mat' Properties.Writable: false Properties.ProtectedLoading: false rd: [64x1 double] Methods