How to put a legend automatically in PCA 3D Scatter Plot?

Here is my solution which successfully creates a 3D scatter plot, but I am unable to add legend to it. I will appreciate any help:

% data = mxn matrix; with all real numbers, no nan, inf or missing value. 
% label = mx1 vector, e.g., label = [0;0;...;0;1;1;...;1;2;2;...;2;3;3;...;3]

myClrMap = [      1 1 0;                      % Yellow
                  1 0 0;                      % Red
                  0 1 0;                      % Green
                  0 0 1;                      % Blue
                  ];

 [coeff_PCA, score_PCA, latent, tsquared, explained, mu] = pca(data);
 
 h = figure();
 
    subplot(1,2,1)    
    scatter3(score_PCA(:,1), score_PCA(:,2), score_PCA(:,3), 15, label, 'filled')
    colormap(myClrMap)    
    class = unique(label);
    
    % Below is my attempt to add legend which does not work.
    [r, c] = size(myClrMap);
    Legend = cell(r,1);
    for i = 1:r 
        Legend{i} = strcat(length(class), num2str(i));
    end
    
    legend(Legend, 'location', 'best')
    
    subplot(1,2,2)
    ...
    ...
    ...   

[Rik]

Original question:

How to put a legend automatically in PCA 3D Scatter Plot?

Here is my solution which successfully creates a 3D scatter plot, but I am unable to add legend to it. I will appreciate any help:

% data = mxn matrix; with all real numbers, no nan, inf or missing value. 
% label = mx1 vector, e.g., label = [0;0;...;0;1;1;...;1;2;2;...;2;3;3;...;3]

myClrMap = [      1 1 0;                      % Yellow
                  1 0 0;                      % Red
                  0 1 0;                      % Green
                  0 0 1;                      % Blue
                  ];

 [coeff_PCA, score_PCA, latent, tsquared, explained, mu] = pca(data);
 
 h = figure();
 
    subplot(1,2,1)    
    scatter3(score_PCA(:,1), score_PCA(:,2), score_PCA(:,3), 15, label, 'filled')
    colormap(myClrMap)    
    class = unique(label);
    
    % Below is my attempt to add legend which does not work.
    [r, c] = size(myClrMap);
    Legend = cell(r,1);
    for i = 1:r 
        Legend{i} = strcat(length(class), num2str(i));
    end
    
    legend(Legend, 'location', 'best')
    
    subplot(1,2,2)
    ...
    ...
    ...   

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[Walter Roberson]

    scatter3(score_PCA(:,1), score_PCA(:,2), score_PCA(:,3), 15, label, 'filled')

That generates one graphical object

   legend(Legend, 'location', 'best')

That can only label graphical objects. You only have one graphical object, so it can only create one legend entry.

Before telling you how to fix the problem within the code you already have:

Have you considered using one of the File Exchange gscatter3() instead ?

[Walter Roberson]

% data = mxn matrix; with all real numbers, no nan, inf or missing value. 
% label = mx1 vector, e.g., label = [0;0;...;0;1;1;...;1;2;2;...;2;3;3;...;3]

myClrMap = [      1 1 0;                      % Yellow
                  1 0 0;                      % Red
                  0 1 0;                      % Green
                  0 0 1;                      % Blue
                  ];

 [coeff_PCA, score_PCA, latent, tsquared, explained, mu] = pca(data);
 
    h = figure();
 
    ax = subplot(1, 2, 1, 'parent', h);
    class = unique(label);
    NC = string(length(class));
    G = findgroups(label);
    hold(ax, 'on')
    splitapply(@(scores,lab) scatter3(scores(:,1), scores(:,2), scores(:,3), 15, lab, 'filled', 'displayname', NC + lab(1) ), score_PCA, label, G);
    colormap(ax, myClrMap)    
    legend(ax, 'show', 'location', 'best')
    hold(ax, 'off');
    xlim(ax, 'auto'); ylim(ax, 'auto')
    
    subplot(1,2,2)
    ...
    ...
    ...   

[Walter Roberson]

It generated 40, 41, 42, 43 for the legend because that is what you asked for. Your code had

    for i = 1:r 
        Legend{i} = strcat(length(class), num2str(i));
    end

Notice the length(class) in there. You have 4 classes, so you are asking that the number 4 be inserted into the legend before each of the four classes. I do not know why you wanted that, so I just reproduced the behaviour.

If you want grid lines and 3D then add in

    grid(ax, 'on');
    view(ax, 3)

[Walter Roberson]

lab is a "dummy variable" name for the anonymous function

@(score_PCA,lab) scatter3(score_PCA(:,1), score_PCA(:,2), score_PCA(:,3), 15, lab, 'filled', 'displayname', NC + lab(1) )

When splitapply(FUN, score_PCA, label, G) is executed, then splitapply() will divide up the input variables score_PCA and label by rows according to the group number in the parameter G. All rows of score_PCA and label that have the same group number will be passed to the function. The selected rows of score_PCA will be passed as the first parameter, and the selected rows of label will be passed to the second parameter.

The anonymous function has to have some name to use for the (subset) of labels that was passed in, so I used the arbitary variable name lab . Because of the way we divided the groups according to label, all of the entries in lab are going to be the same, but there will be one row for each row in the selected rows from score_PCA .

I probably should also have used a different variable name for the first parameter, such as

splitapply(@(sP,lab) scatter3(sP(:,1), sP(:,2), sP(:,3), 15, lab, 'filled', 'displayname', NC + lab(1) ), score_PCA, label, G);

to emphasize that what is received as the first parameter is not the full set of score_PCA values.