How do I add a legend to a boxplot in MATLAB?

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I created a boxplot and would like to add a legend to it, but when I call legend I get the following warning message and there is no legend in the plot figure: 
 
Warning: Plot empty.
> In legend at 286
 
Is it possible to add a legend to a box plot? My code is: 
 
 
figure;
colors = [1 0 0; 1 0 0; 0 0 1; 0 0.5 0; 0 0.5 0; 0 0.5 0];
x = boxplot(rand(100,6),'Colors',colors);
legend('Group A','Group B','Group C')

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 25 Apr 2014
Box plots in the Statistics Toolbox do not support legends as of release R2014a.
As a workaround you can explicitly pass to the "legend" function the handle to the axes in which the box plot is drawn:
figure;
colors = [1 0 0; 1 0 0; 0 0 1; 0 0.5 0; 0 0.5 0; 0 0.5 0];
x = boxplot(rand(100,6), 'Colors',colors);
% findall is used to find all the graphics objects with tag "box", i.e. the box plot
hLegend = legend(findall(gca,'Tag','Box'), {'Group A','Group B','Group C'});
Depending on the number of variables that you have, you might not obtain the right color for each legend element. Use the following trick to manually change the color of each legend element:
% Among the children of the legend, find the line elements
hChildren = findall(get(hLegend,'Children'), 'Type','Line');
% Set the horizontal lines to the right colors
set(hChildren(6),'Color',[1 0 0])
set(hChildren(4),'Color',[0 0 1])
set(hChildren(2),'Color',[0 0.5 0])
  2 Comments
Noname
Noname on 18 Dec 2016
Edited: Noname on 18 Dec 2016
Thanks a million! I am student, your question and answers give me great help.Thanks a million!
% and I find findobj() can replace findall().
box_vars = findobj(gca,'Tag','Box');

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