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

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')

Answer 1

[MathWorks Support Team]

As of MATLAB R2020a, "boxchart" is an alternative to "boxplot" and allows the creation of a legend. The following documentation illustrates some of the advantages of a "boxchart":

https://www.mathworks.com/help/stats/boxplot.html#mw_c2e3616d-49fc-40af-80f9-c6655ccf8f8b

The following documentation demonstrates the use of "boxchart":

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

For MATLAB R2014a and prior, 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])

In R2014b and after, the Children property of Legend does not return the line plots. A workaround is as follows:

box_vars = findall(gca,'Tag','Box');
hLegend = legend(box_vars([3,2,4]), {'Group 1','Group 2','Group 3'});

[Dyuman Joshi]

Sebastian Lopez Saavedra (I can't seem to tag the profile directly) notes -

"This reply has to be updated because it doesn't work in Matlab 2023"

[Dyuman Joshi]

As an alternative, you can use the boxchart function, which does support the functionality to directly use legends.