Why does legend command fail after boxplots with hold command?

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Following the MATLAB legend documentation example , the following code excerpt issues the warning and erroneous image displayed in the screenshot. Why? How do I create a legend?
cm3tomm3 = 10^3;
datatoplot = ExtractLessData(height(data),newvoldata,'blurred','rectum');
figure
boxplot(cm3tomm3*datatoplot.Volumes,datatoplot.DoseInterval,...
'colors','b','BoxStyle','filled')
title('Dosed Rectal Volumes (mm^3) per Dose Interval (Gy(RBE))')
hold on
datatoplot = ExtractLessData(height(data),newvoldata,'planned','rectum');
boxplot(cm3tomm3*datatoplot.Volumes,datatoplot.DoseInterval,...
'colors','r')
hold off
legend('planned: red, outlined', 'blurred: blue, filled')

Answers (1)

Elizabeth Reese
Elizabeth Reese on 30 Nov 2017
Boxplots are groups of lines in the axes, so their legend functionality is limited. Mathworks' teams are aware of this and considering improving this in a future release.
You can still achieve this legend by saving the handles from the boxplots and then manually creating the legend.
For example:
load carsmall
% Make the plot
f = figure;
h1 = boxplot(MPG,Origin,'colors','b','BoxStyle','filled');
set(gca,'XTickLabel',{' '})
hold on
h2 = boxplot(gca,Acceleration,Origin,'colors','r');
hold off
% reset the axes limits to show the full boxplots
ylim([min(min(MPG),min(Acceleration)) max(max(MPG),max(Acceleration))])
legend([h1(2),h2(3)],'one, blue filled','two, red, outlined')
You will need to find which index in h1 and h2 correspond to lines in the box of your plot.
  1 Comment
Daniel Bridges
Daniel Bridges on 5 Dec 2017
Edited: Daniel Bridges on 5 Dec 2017
> You will need to find which index in h1 and h2 correspond to lines in the box of your plot.
How do I do this? I assigned the boxplots to variables as you did h1, h2 and my results are 4x19 and 7x19 double arrays. It's not clear what these arrays represent ( plot visualizes them as straight lines); I checked the boxplot documentation and it doesn't mention the case of assigning boxplot to a variable (i.e. h1 = boxplot(...)).

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