How do I create a legend and include loop variable values in the labels?

I created the matrix P with 400 rows, 3 columns (which I call X, Y and Z).

The variables in each row are related by a simple quadratic (Y^2) and intercept term (X):

Z = Y^2 + X

And I created 10 curves, one for each unique value of X with the following code starting by isolating each unique value of X with "PU = unique..."

Please Note: This entire code only takes about 5 seconds to run:

    P = [];
    for X = 1:1:10
    for Y = 0.1:0.1:4
    Z = (Y^2) + X;
    P = vertcat(P, [X, Y, Z]);
    end
    end

    PU = unique(P(:,1)); 
    PUC = size(PU, 1);
    for i = 1:PUC;
    Q{i} = P(P(:, 1) == PU(i), :); 
    q = Q{i};
    x = PU(i);
    plot(q(:, 2), q(:, 3))
    legend(.......which code goes here??...)
    hold on
    end
    hold off

Each line looks the same, and there are no labels. How do I label each line with using the variable x = PU(i) (which runs from 1 to 10) in a legend which will look like: "X = 1", "X=2", "X=3" etc... up to "X = 10", and have a changing colour or line style.

Thanks

D Howard

Answer 1

[Kevin Holst]

When you create a plot, you can specify the legend labels by setting the “DisplayName” property as name-value pair. Set the "DisplayName" property to a character vector of the text that you want to include in the legend. To include a variable value in the text, use “num2str”.

For example:

    hold on
    for k = 1:10
        txt = ['X = ',num2str(k)];
        plot(rand(10,1),'DisplayName',txt)
    end
    hold off
    legend show

Starting in R2014b, plotted lines cycle through the colors in the color order.

[Doron]

Is there any way to just to get Matlab to change the plot style itself successively?

[Doron]

Thanks for the answer Kevin, I went with this solution, works fine, but would be nicer not to have to do this.

[Amalia]

Very useful! Thank you!

[Shubham Maurya]

This worked for me. Thanks

[Surya]

Perfect!

[Shaghayegh Javadi]

this is very helpful :)

[Astrid Edelman- McCabe]

Kevin you are a legend

[Pief]

Thanks Kevin, and I agree with the pun Astrid made

[Benjamin]

easy and to the point. Thank you, but I have a problem with it and that is when I added this section to my code the colors of the legends are not consistent with the colors of the figures. In fact, the color of of the legend changes for each curve but the color of each curve doesn't change!!

[Prateek Nagvani]

Thanks Kevin!

[Eldwin Cheung]

If u dont want to call a variable, u can just put the name directly like so:

plot(X,Y,'DisplayName','Name')

[Shivam Chopra]

Thanks man.. it helped a lot

[Kris Hoffman]

sprintf works well here also.

[Ashfaq Ahmed]

It was super helpful! Thanks!

[Nasim Hasanpour]

Thanks. It was so helpful.

Does anyone know how to change the legends' font in this code? I want to use the plot in Latex.

[Furkan Sencer Kaçar]

@Nasim Hasanpour Have you got the answer of your question?

Answer 2

[Kevin Holst]

Not in the way that you're currently doing it, but if you're not married to the idea of using your P variable how you currently are using it, I'd suggest something like this:

P = (0.1:0.1:4)';
for i = 2:11
    P(:,i) = P(:,1).^2 + (i-1);
    legendInfo{i-1} = ['X = ' num2str(i-1)]; 
end
plot(P(:,1),P(:,2:end))
legend(legendInfo)

The problem with this is that Matlab cycles through 7 different colors, but doesn't switch the line type automatically when it gets through the colors. So the first 3 and last 3 curves will be the same colors. This way does speed up your code quite significantly.

[Kevin Holst]

I found a way to fix this! :)

set(0,'DefaultAxesLineStyleOrder',{'-','--',':'}) %or whatever order you want

This is why I help out on Answers, I learn new useful things! :D

[Saad Rabbani]

Hi, im trying to make a legend for 4 plots that are inside a for loop but then i have an additional plot outside the loop as well. The plot either shows the legend of the 4 plots inside the loop or the one plot i have outside. Im not sure how i can make a legend for all five plots.

[Giuseppe Degan Di Dieco]

Dear Kevin,

thanks for your tip, it really helped me in setting the labels of a legend in a for loop.

Hope that in 2021, you're still reading the community updates.

Best.

Answer 3

[Elgyii]

% You can simply store all the handles while you plot, for example:

For i = 1 : 10 h(i) = plot(x,y) end legend(h, names) % names should be what you want to write out on legend

[Soroush Shahnia]

Thanks Eligio. The first solution is killing my laptop and it's very slow. This works much faster for me.

Answer 4

[amir king]

I still have problem with the legend. The problem is all the legend are the same line-style. What is missing or what is wrong?please help

this is my code:

    for u=1:5   % dfferent U infinity  
     colorstflex = 'kbgrm';
     figure(u); cla;  %%for diffrent RE
     plotStyle = {'-o','-*','-r.','-s','-^'};
     hold on
     for k=1:3   
     Vel_rel(Vel_rel == 0) = NaN;
     x_Dmat = repmat(x_D,[5 1]);
     plot(x_Dmat,Vel_rel(u,:,k),plotStyle{k},'color',colorstflex(k));hold on
     legendInfo{k}=['flex no. ', num2str(k)]); % or whatever is appropriate
     legend(legendInfo)
end 
end

[Elgyii]

% You can simply store all the handles while you plot, for example:

For i = 1 : 10
h(i) = plot(x,y)
end
legend(h, names)
% names should be what you want to write out on legend

[Karl]

So helpful! Many thanks,

[mahdi fatehpour]

thanks.

Answer 5

[Brian Sutton]

I had a similar problem, to create a legend, but not knowing how many plots I would have. I wanted to set the legend dynamically. I looked at answers using the "DisplayName" property while plotting, but I didn't want to rewrite all of my plotting, and in any case ewasn't sure how to use it with plotyy (probably using hax = plot / semilogy construct). Anyway, I managed to do it by adding to the cell array that contains the legend text in a loop related to how many plotlines I might have. I would have either 5 or 4 to start with (depending on a variable status) and up to "L" plots for v"i" Active followed by up to L plots for v"i" Serious, where "i "would be a label (not a vector component) for each legend item, as follows (I left semicolons off end-of-lines for testing, to print what I was creating). This worked very well. The solution was at https://octave.org/doc/v4.2.2/Basic-Usage-of-Cell-Arrays.html although usually I find MatLab (Octave in my case) documentation too arcane.

if (vac_per_day(1) !=0 || vac_eff(1) != 0) %20210314

legend_text = {['All cases'], ['Healthy'], ['All infected'], ['Deaths'], ['Vaccinated']} %5 items

for i = 1:L

legend_text{5+i} = strcat("v", num2str(i), " Active")

legend_text{5+L+i} = strcat("v", num2str(i), " Serious")

endfor

else

legend_text = {['All cases'], ['Healthy'], ['All infected'], ['Deaths']} %4 items

for i = 1:L

legend_text{4+i} = strcat("v", num2str(i), " Active")

legend_text{4+L+i} = strcat("v", num2str(i), " Serious")

endfor

endif

legend = legend(legend_text, 'location', 'southoutside', 'orientation', 'horizontal');

set(legend, 'Interpreter', 'none') %20201218 to stop underscore being lower case if used

Producing (not a pretty chart but) the legend exactly as I wanted (apart from overflowing the box). You can guess this is all to do with Coronavirus modelling. The model numbers are not the realistic UK model, that uses other input data. This is a test harness for calibrating age related subgroups (in other charts)