How to add a second legend-box to a figure without new plots?

Hey everyone, I already looked into the answers of the MATLAB-Community that were given to questions very similar to mine (https://de.mathworks.com/matlabcentral/answers/367161-2-legends-in-1-plot; https://de.mathworks.com/matlabcentral/answers/365006-how-to-create-2-legends-in-one-figure)

I tried these solutions, but they didn't work for me. That's why I am asking for help here.

To describe my problem, here are the facts:

• I want to plot several functions, some are shown as solid lines and some as dotted lines
• I added a legend to name these lines by color
• Now I want to explain what the normal, solid lines are standing for and what the dotted-lines are standing for
• I want to do this by adding a second legend (in an own "legend-window") to the figure

As far as I know, you can only have one legend-window for one set of axes in MATLAB, so the idea is:

• add a second (exatly equal) set of axes to the figure
• make this axes invisible, so you don't see it later in the plot
• add two "helping - lines", one solid and one dotted
• make these helping - lines also invisible
• add a second legend to describe the helping - lines

As you can see in the following code, I made an example of how I tried to find a solution for my problem with simple functions.

It does't work good, because the second set of axes ('secondaxes') is not invisible and the second legend (the one with 'Model' and 'Catalogue' in it) is not black, but grey, because I had to make the helping lines H1 and H2 invisible...

clear all 
% First Line to plot: 
t = 0:0.5:5;
f = (2*sin(2*60*t));
% "Helping Lines", for example k and r: 
p = 0:0.5:5;
k = 2*p + 3;
o = 0:0.5:5;
r = 2*o +3.2;
%%%%%%%%% First plot (L1) and the axes for that plot (firstax) %%%%%%%% 
f1 = figure;
firstax = axes (f1, 'FontSize', 16); 
hold on 
xlim ([0 100]);
xlabel('Current [A]');
ylabel('Voltage [V]');
L1 = plot (f, '-', 'LineWidth', 2, 'Color', [0 84 159]/255, 'Parent', firstax);
hold off 
set(firstax, 'Box', 'off')
legend(firstax, L1, '4 A', 'Location', 'southeast');
%%%%%% axes for the second plot (secondaxes) and the two helping Lines H1 and H2 %%%%%%%%
hold on 
secondax = copyobj(firstax, gcf);
delete( get(secondax, 'Children'))
H1 = plot(k, '-', 'LineWidth', 2, 'Color', [0 0 0]/255, 'Parent', secondax, 'Visible', 'off');
H2 = plot(r, '--', 'LineWidth', 2, 'Color', [0 0 0]/255, 'Parent', secondax, 'Visible', 'off');
set(secondax, 'Color', 'none', 'XTick', [], 'YAxisLocation', 'right', 'Box', 'Off') 
legend (secondax, [H1 H2], 'Model', 'Catalogue', 'Location', 'northeast');
hold off

Answer 1

[Cris LaPierre]

Do you want to have two different scales on the y axes?

Modifying the placement of holds and labels seemed to fix seeing the double axes. I assume at some point you will be adding real data for H1 and H2 that won't be hidden, so that legend will appear the same. The background color can be changed by adding a Color name-value pair to the legend command.

...
%%%%%%%%% First plot (L1) and the axes for that plot (firstax) %%%%%%%% 
f1 = figure;
firstax = axes (f1, 'FontSize', 16); 
L1 = plot (f, '-', 'LineWidth', 2, 'Color', [0 84 159]/255, 'Parent', firstax);
set(firstax, 'Box', 'off','XLim',[0 100]);
legend(firstax, L1, {'4 A'}, 'Location', 'southeast');

%%%%%% axes for the second plot (secondaxes) and the two helping Lines H1 and H2 %%%%%%%%
hold on 
secondax = copyobj(firstax, gcf);
delete( get(secondax, 'Children'))
H1 = plot(k, '-', 'LineWidth', 2, 'Color', [0 0 0]/255, 'Parent', secondax, 'Visible', 'off');
H2 = plot(r, '--', 'LineWidth', 2, 'Color', [0 0 0]/255, 'Parent', secondax, 'Visible', 'off');
set(secondax, 'Color', 'none', 'XTick', [], 'YAxisLocation', 'right', 'Box', 'Off') 
legend ([H1 H2], {'Model', 'Catalogue'}, 'Location', 'northeast');
hold off

xlabel('Current [A]');
ylabel('Voltage [V]');

[Fabian Binz]

Thank you very much for your quick response!

I wanted to have online one scale, the second one was more like a 'must have', because a legend without own axes is impossible in MATLAB...

I looked into the placement of holds and I think I found the mistake there...

Here is my solution that I used for my problem and the correct plot:

...
f1 = figure();
Ax(1) = axes(f1); 
xlabel('Current [A]');
ylabel('Voltage [V]');
hold on 
L1 = plot (f, '-', 'LineWidth', 2, 'Color', [0 84 159]/255, 'Parent', Ax(1));
set(Ax(1), 'Box','off')
legend(Ax(1), L1, '4 A', 'Location', 'southeast');
%%%%%%%%%copy axes 
Ax(2) = copyobj(Ax(1),gcf);
delete( get(Ax(2), 'Children') )
hold on
%%%%%%%%plot helping lines
H1 = plot(k, '-', 'LineWidth', 2, 'Color', [0 0 0]/255, 'Parent', Ax(2));
H2 = plot(r, '--', 'LineWidth', 2, 'Color', [0 0 0]/255, 'Parent', Ax(2));
%'Visible', 'Off'
hold off
set(Ax(2), 'Color', 'none', 'XTick', [], 'YAxisLocation', 'right', 'Box', 'Off', 'Visible', 'off') %make it transparent
legend ([H1 H2], 'Model', 'Catalogue', 'Location', 'northeast');

[Leone Campos]

If you want the second legend to be black instead of greyed out, you can plot dummy points and keep them visible:

H1 = plot(NaN, '-', 'LineWidth', 2, 'Color', [0 0 0]/255, 'Parent', Ax(2));
H2 = plot(NaN, '--', 'LineWidth', 2, 'Color', [0 0 0]/255, 'Parent', Ax(2));

[Davide Zamblera]

You can change the legend color by adding the corresponding attribute:

legend ([H1 H2], 'Model', 'Catalogue', 'Location', 'northeast', 'Color',[1 1 1]);

Answer 2

[Hamdullah Livaoglu]

Another approach is here; (R2015b)

characterizing multi properties for its legends in the same figure ; (legendTitle function is derived from https://nl.mathworks.com/matlabcentral/fileexchange/48331-add-a-title-to-a-legend)

hold on
col = rand(length(hv),3);
for j = 1 : length(hv)

   p(j)=plot(sPeriod,hv{j},'Color', col(j,:),'LineWidth',1.2);
   p1(j)=plot(sPeriod,hv{j},'Color', col(j,:),'LineWidth',1.2);
   
end


xlabel('Predominant Periods (s)','FontWeight','bold');
ylabel('Response Spectral Ratios (RHV)','FontWeight','bold');
set(gca, 'XScale', 'log');
set(gca, 'XLim', [0.05 2]);
set(gca, 'XTick', [0.05 0.1 0.2 0.4 0.6 0.8 1.5 2]);
hold off

leg1=legend(p,num2str(round(R(ind)')),'Location','NorthEast');set(leg1,'FontSize',9);
ah1=axes('position',get(gca,'position'),'visible','off');
leg2=legend(ah1,p1,magnitude{ind},'Location','NorthWest');set(leg2,'FontSize',9);
legendTitle (leg1, 'R (km)' );
legendTitle (leg2, 'Magnitude' );

[David Walwark]

Worked for me, thank you

[Sedevizo Kielienyu]

Thank you so much! I have been looking everywhere to display two legends. Your method is the only one that worked for me.

[Wei Zhao]

The best solution! Thank you. 👍😀

Answer 3

[Anna]

Here's a minimal working example with more than one line (works on R2017b):

%% define data
% data
x = 0:0.5:5;
y1 = (2*sin(2*60*x));
y2 = (2*cos(2*60*x));
y3 = (2*sin(60*x));
y4 = (2*cos(60*x));

% helper data (as many as different line styles are required) 
h1 = 2*x + 3;
h2 = 2*x +3.2;

%% create plot
f1 = figure();
Ax(1) = axes(f1); 
xlabel('x data');
ylabel('y data');
hold on 
Y1 = plot(x,y1, '-', 'LineWidth', 2, 'Color', [0 0 1], 'Parent', Ax(1));
Y2 = plot(x,y2, '-', 'LineWidth', 2, 'Color', [1 0 0], 'Parent', Ax(1));
Y3 = plot(x,y3, '--', 'LineWidth', 2, 'Color', [0 0 1], 'Parent', Ax(1));
Y4 = plot(x,y4, '--', 'LineWidth', 2, 'Color', [1 0 0], 'Parent', Ax(1));
% do NOT hold off here!
set(Ax(1), 'Box','off')

% add colour legend
lgd1 = legend(Ax(1), [Y1 Y2],'blue','red', 'Location', 'southeast');  % only select first two data lines, as there's only two different colours
title(lgd1,'Colours') % add legend title

% copy axes 
Ax(2) = copyobj(Ax(1),gcf);
delete(get(Ax(2), 'Children') )

% plot helper data, but invisible
hold on
H1 = plot(x,h1, '-', 'LineWidth', 2, 'Color', [0 0 0], 'Parent', Ax(2),'Visible', 'off');
H2 = plot(x,h2, '--', 'LineWidth', 2, 'Color', [0 0 0], 'Parent', Ax(2),'Visible', 'off');
hold off

% make second axes invisible
set(Ax(2), 'Color', 'none', 'XTick', [], 'YAxisLocation', 'right', 'Box', 'Off', 'Visible', 'off')

% add linestyle legend
lgd2 = legend([H1 H2], 'Linestyle 1', 'Linestyle 2', 'Location', 'northeast');
title(lgd2,'Linestyles') % add legend title
set(lgd2,'color','none')

[Leone Campos]

Nicely done!

If you want the second legend to be black instead of greyed out, you can plot dummy points and keep them visible:

H1 = plot(NaN,NaN, '-', 'LineWidth', 2, 'Color', [0 0 0], 'Parent', Ax(2));
H2 = plot(NaN,NaN, '--', 'LineWidth', 2, 'Color', [0 0 0], 'Parent', Ax(2));