Creating a Legend for a Boxplot with Two Colors

Hi, I need help inserting a legend for a boxplot with two colors (red & blue). My code for this is:

boxplot(data, 'notch', 'on', 'symbol', '*','colors','rb');

legend('Label1','Label2');

But I get no legend and it says "Warning: Plot empty" in the command window. Any help on this matter would be much appreciated. Thank you.

Answer 1

[Walter Roberson]

It isn't a bug. The annotations for the boxes have their IconDisplayStyle property set 'off' which is a signal that they are not to be included in legends.

You can include them deliberately by using

legend(findobj(gca,'Tag','Box'),'Label1','Label2')

but note that the boxes all have the same color (at least with the call you use) so the legend lines will be in the same color as each other.

[Danny]

Thank you, that did exactly what I wanted it to. Actually the call I used ('colors','rb') makes every odd numbered box red and every even numbered box blue. Thanks again for such a quick response.

[Oleg Komarov]

@Walter: enlightening as usual.

[Billy]

Is there a way to make the legend lines have different colors in this case, e.g. when using: legend(findobj(gca,'Tag','Box'),'Label1','Label2')?

[Isabel Chen]

I am using Matlab 2016a, and with the code above, I get the following error message:

Warning: Ignoring extra legend entries. 
> In legend>set_children_and_strings (line 643)
  In legend>make_legend (line 328)
  In legend (line 257)

The plot looks fine, apart from the legend entries in the wrong order. Any ideas?

[Walter Roberson]

In your line similar to

legend(findobj(gca,'Tag','Box'),'Label1','Label2')

you need to supply as many labels as you have columns in your data. boxplot creates one object with tag Box for each column of data.

Answer 2

[Oleg Komarov]

Use the option labels, dunno if it is a bug but legend doesn't seem to work.