How to create a "contour" plot with a varying X and Y vector values?

Question

Oscar Aguilar-Cerna 2023-5-11

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回答： Walter Roberson 2023-11-3

I am trying to plot a 3rd variable on an x and y plane, and would like to make it into a contour, specifically with the contourf(x,y,z) function. Unfortunately, my x and y values are not in strictly increasing or decreasing order, and there are repeated values, which the contourf function does not accept. I cannot use the sort function for my x and y vectors as they are associated with each other, and the sort function will mess up their pairings. Is there another function that is similar to contourf that will allow my x and y vectors? I have already looked into the pcolor and contour functions, but they are not giving me results that I would like. Basically, I want to have my x and y values in any order, and for repetition to be allowed. I cannot share my raw data as it is confidential.

Below is the scatter plot version. I want this plot to simply be a contour version of this scatterplot, with a colorbar to see the variance in the 3rd variable as a function of the other two variables.

Here is a sample of my code:

%%creating a circle
%// radius
r = 0.38;
%// center
c = [0 0];
pos = [c-r 2*r 2*r];
rectangle('Position',pos,'Curvature',[1 1])
hold on
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
dz = diag(z); %creating my z vector into a diagonal matrix
%scatter(x,y,z,'filled'); %previous plot, I want to convert it into a
%contour plot
contourf(x,y,dz); % contourf only takes my 3rd variable as a matrix, I am not sure if
%this diagonal vector will work for my needs. 
title('Velocity Distribution for Natural Gas Injector');
xlabel('X position [inches]');
ylabel('Y Position [inches]');
a=colorbar;
ylabel(a,'Velocity [ft/s]','FontSize',10,'Rotation',270);
a.Label.Position(1) = 0.95;
clim([110 140]);
axis equal
hold off

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Answer 1

Mann Baidi 2023-8-28

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编辑：Mann Baidi 2023-11-3

Hi Oscar,

I understand that you are facing issue in using the “contour” function as you don’t have the vectors in increasing order. I would suggest you use the “plot3” function.

You can find the documentation of the function below:

https://www.mathworks.com/help/matlab/ref/plot3.html

Hope this helps!

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Answer 2

Walter Roberson 2023-11-3

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contourf() is not applicable to volumes. To contour on volumes, use isosurface -- usually with transparency so you can see "inside" the shells.

You cannot create meaningful contours when there are duplicate spatial points with different f values: the existence of such points would imply that that one point was simultaneously every value between the min() and max() of the f at that point, since contouring has the implicit assumption of continuity.