Why does legend command fail after boxplots with hold command?

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')

Answer 1

[Elizabeth Reese]

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.

[Daniel Bridges]

> 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(...)).