MATLAB: How to add a legend to a boxplot in MATLAB

boxplotlegendStatistics and Machine Learning Toolbox

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')

Best Answer

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])
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