How to set a legend with a label for two or more lines?

Hello,

In the graphic legend I want to share the same label for two or more lines.

For example, I have two (or more) pair of data

x = linspace(0,0.5);
for i = 1:3
    y1(i,:) = x.^2 + i;
    y2(i,:) = (2*x).^2 + i;
end

My plot instructions are

hold on
plot(x,y1,'-')
set(gca,'ColorOrderIndex',1)
plot(x,y2,'--')
legend({'label 1','label 2','label 3'})

I want that the first curves of y1 and y2 share the label “label 1” and so on for the second and the thirds lines, “label 2” and “label 3”, respectively. Something like this

Thanks in advance for the answers.

[Walter Roberson]

Same technique as I happened to answer earlier today in

https://www.mathworks.com/matlabcentral/answers/504259-how-to-modify-color-of-legend-markers#answer_414385

[Lemuel Carlos Ramos Arzola]

Dear Walter,

Thanks for your comment, but I want this

Sorry if my question was not clear.

[Walter Roberson]

Ah. In that case, you will need to construct your own legend box. legend() is not designed to be able to have more than one linestyle (or color) for any given label.

[Lemuel Carlos Ramos Arzola]

So sad,

Any advice (links) about how can I do that?

[Walter Roberson]

https://www.mathworks.com/matlabcentral/fileexchange/31092-legendflex-m-a-more-flexible-customizable-legend makes a good starting point.

[Adam Danz]

If the solid and dashed lines represent the same thing within each color, why not just show the solid lines in the legend?

hold on
h = plot(x,y1,'-'); 
set(gca,'ColorOrderIndex',1)
plot(x,y2,'--')
legend(h, {'label 1','label 2','label 3'})

If the dashed lines must be included in the legend, IMO having duplicate labels would be easier than construction your own legend.

[Lemuel Carlos Ramos Arzola]

Dear Adam,

The solid and the dashed lines are from different models (eg., newtonian and non-newtonian fluid models), both are mandatory in the legend. Of course, having duplicate labels would be easier, but space-consuming in the graphic, especially when we have many curves (see my second figure for example).

[Adam Danz]

Matlab needs to have an option for grouped symbols/linestyles/colors in the legend. But it currently doesn't. In these cases I've done something like this. It defines the colors and line styles as groups.

h = plot(x,y1,'-'); 
set(gca,'ColorOrderIndex',1)
plot(x,y2,'--')
h2 = plot(nan, nan, 'k-'); 
h3 = plot(nan, nan, 'k--'); 
legend([h;h2;h3], {'label 1','label 2','label 3', 'Newtonian',' Non-Newtonian'})

[Lemuel Carlos Ramos Arzola]

Excellent alternative solution.

Thanks

(but the actual problem is unsolved)

[Adam Danz]

I understand. Walter pointed you to a function on the file exchange that might be helpful in building a custom legend.

Answer 1

[Jeremy]

This isn't a particularly elegant solution, but I think it does what you want.

x = linspace(0,0.5);
for i = 1:3
    y1(i,:) = x.^2 + i;
    y2(i,:) = (2*x).^2 + i;
end
hold on
plot(x,y1,'-')
set(gca,'ColorOrderIndex',1)
plot(x,y2,'--')
[lh, labelhandles] = legend({'label 1','label 2','label 3'});
labelhandles(5).LineStyle = '--';
labelhandles(4).YData = [0.83 0.83];
labelhandles(5).XData = [0.0460 0.5057]; labelhandles(5).YData = [0.760 0.760];

This will only update one of the legend entries, I trust you can extend this solution to the others. You could also choose a labelhandles variable with a shorter name to make it look a little neater, if you wanted.

Your YData property can be tweaked until you are happy with the appearance.

[Adam Danz]

+1; Good idea, Jeremy!

One small note of caution: since r2018b only the first output of the legend function is documented. Archived documentation for the legend function from r2016a to r2018a suggests avoiding the use of the outputs other than the first output since it "... creates a legend that does not support all graphics features".

For example, when the first and second outputs to legend() are both included, it interferes with the ability to change the fontsize of the legend text.

But it's simple and solves the problem. It might be wise for Lemuel to add a reminder-comment near this section of code in case there's weird problems in the future.

[Lemuel Carlos Ramos Arzola]

Very good idea Jeremy!

A little bit complex but it tackled the problem. I will be working with the legend function outputs to generalize your idea in my actual code.

I hope in the future Matlab automates this issue.

Thanks a lot.

Dear Adam thanks for the "note of caution", very important too.

I love Matlab and now I also love the community.

Best,

Lemuel

[Adam Danz]

Good to hear!

Answer 2

[Laurens]

I understood the problem as follows: One legend entry can correspond to many lines on the graph. But Matlab assigns the first legend entry to colors/markers of the first line on the graph, the second entrie's colors/markers correspond to the second line in the graph and so on.

My solution therefore was to tell Matlab exactly - for each group of lines - which line belongs to which entry. you only have to do this once for each legend entry.

Exemple code:

figure

p1 = plot(x, y3, 'LineWidth', 0.3, 'Color', [0.4, 0.4, 0.4, 0.05]);

hold on

p2 = plot(x, y1, 'LineWidth', 2, 'Color', [1, 0, 0, 0.1]);

hold on

p3 = plot(x, y2, 'LineWidth', 5, 'Color', [0.9, 0, 0, 1]);

legend([p1(1,:); p2(1,:); p3(1,:)], {'entry 1','entry 2', 'entry 3' }, "Location","southoutside")

p1(1,:) samples one line (the first) from the first group of lines

p2(1,:) samples one line from the second group of lines

and so on ...