Simplest way to add a legend to the groups of a scatter plot

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the cyclist on 4 Feb 2013
Commented: Anton on 9 Apr 2014
Here is a trivial scatter plot that uses the default marker size (S=[]) and a vector C to assign three colors according to the current colormap.
M = magic(3);
S = [];
C = [1;1;1;2;2;2;3;3;3];
figure
scatter(M(:),sort(M(:)),S,C,'filled')
Is there a simple way to create a legend to the three colors?

bym on 5 Feb 2013
Edited: bym on 5 Feb 2013
gscatter(M(:),sort(M(:)),C) the cyclist on 5 Feb 2013
Yet another MATLAB function that I knew and then forgot. Thanks for the reminder!

ChristianW on 5 Feb 2013
Without gscatter fcn:
[~, I] = unique(C);
p = findobj(gca,'Type','Patch');
legend(p(I),'X','Y','Z')
Anton on 9 Apr 2014
I'd like to add a correction to this solution, since it doesn't work properly in my case (8.2.0.701 (R2013b)). The problem appears, when the number of elements corresponding to different colors is not the same, for instance:
C = [1;1;1;1;2;3;3;3;3];
As far as I understand, order of handles provided by findobj is inverted: point corresponding to the first element of M has the last position in the array p.
I suggest the following solution:
[~, I] = unique(C); %
I = length(C) - I(length(I):-1:1);
p = findobj(gca,'Type','Patch');
legend(p(I),'X','Y','Z');