MATLAB Answers

the cyclist

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

Asked by the cyclist
on 4 Feb 2013
Latest activity Commented on by Anton
on 9 Apr 2014
Accepted Answer by bym

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?

  0 Comments

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2 Answers

bym
Answer by bym
on 5 Feb 2013
Edited by bym
on 5 Feb 2013
 Accepted answer
gscatter(M(:),sort(M(:)),C)

  1 Comment

the cyclist
on 5 Feb 2013

Yet another MATLAB function that I knew and then forgot. Thanks for the reminder!


Answer by ChristianW
on 5 Feb 2013

Without gscatter fcn:

[~, I] = unique(C);
p = findobj(gca,'Type','Patch');
legend(p(I),'X','Y','Z')

  1 Comment

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');

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