Adding legend for subplot when none of the subplots include all legend entries

Hi folks, I am trying to create a subplot where the points are coloured by year, and the years have the same colour in each subplot. Through careful control of the colours I have been able to achieve this, but I only want one legend for the figure, and none of the subplots contain all the years. I cannot figure out (excuse the pun!) how to create just one legend that has all the years (2011-2016). I can easily position one legend at the bottom of the subplot, that's not the problem. Here is the code to make the figure with individual legends (which I don't want in the end) to show you what I mean, along with the code. Thank you!

st_colours=[0, 0.4470, 0.7410;0.8500, 0.3250, 0.0980; 0.9290, 0.6940, 0.1250; 0.4940, 0.1840, 0.5560; 0.4660, 0.6740, 0.1880; 0.3010, 0.7450, 0.9330; 0.6350, 0.0780, 0.1840];
figure15 = figure();
for m=1:12
    subplot(4,3,m)
    if m==1 || m==2 || m==3 || m==4  || m==5 || m==6 || m==8 || m==11  || m==12  %Month
        for r=1:5 %The row this time
        sc(m,r)=scatter(QQG_Amm3M_T_split(m).months.NOAA_ONI(r),QQG_Amm3M_T_split(m).months.mean_Tmelt(r),25,'filled');hold on;
        end
    end
    if m==7 || m==9 || m==10
        for r=1:4 %The row this time
        sc(m,r)=scatter(QQG_Amm3M_T_split(m).months.NOAA_ONI(r),QQG_Amm3M_T_split(m).months.mean_Tmelt(r),25,'filled');hold on;
        end
    end
    if m==11 || m==12 
        for r=1:5 %The row this time
        sc(m,r).MarkerFaceColor=st_colours(r,:);
        end
    elseif m==1 || m==2 || m==3 || m==4  || m==5 || m==6 || m==8
        for r=1:5 %The row this time
        sc(m,r).MarkerFaceColor=st_colours(r+1,:);
        end
    else
        for r=1:4 %The row this time
        sc(m,r).MarkerFaceColor=st_colours(r+1,:);
        end
    end
    legend(string(QQG_Amm3M_T_split(m).months.mean_Year));
    title(Month_labels(m));
    xlim([-2 2.5]);
    ylim([0 1]);
end

[Adam Danz]

You can simplify the if m==1 || m==2 || m==3 || m.... conditions by using ismember instead.

if ismember(m, [1,2,3, . . .])

[Catriona]

That's handy, thanks Adam!!

Answer 1

[Adam Danz]

All you need are the handles to each object you want to include in the legend and the handles can come from different subplots within the same figure. But the legend is a child of only one subplot and can be repositioned relative to that subplot.

% DEMO
figure()
colors = [winter(3);summer(3);autumn(3);copper(3)]; 
subPlotNames = 'ABCD'; 
h = gobjects(3,4); 
for i = 1:4
    ax = subplot(1,5,i);
    h(:,i) = plot([0,1],[.66;.5;.33].*[1,1],'LineWidth',4);
    text([.1,.1,.1],[.66;.5;.33],{'1' '2' '3'},'VerticalAlignment', 'Bottom')
    set(h(:,i), {'DisplayName'}, compose('%s %d',subPlotNames(i),1:3)')
    title(subPlotNames(i))
end
set(h(:), {'Color'}, mat2cell(colors,ones(12,1),3))
lg  = legend(h(:)); 
lg.Position(1:2) = [.8 .3];

This method also works with TiledLayout but requries r2020b for the last line which repositions the legend.

Another method I've used a lot is to use the DisplayName property of line objects to assign the legend string to all line objects within a figure even if the legend string & line line properties associations are redundant (e.g. more than 1 orange marker named "2012"). Then use legendUnq() from the file exchange to eliminate the objects with redundant legend strings. After applying DisplayName, you just need to call this after assigning all objects to the plot,

legend(legendUnq())

[Catriona]

Cool that's super interesting! My dinner is ready otherwise I would have tried it now but I will check with my code first thing :)

[Catriona]

Just to say that after a bit of tweaking I was able to use Adam's idea to get one legend with all the entries I needed. The key is to set the display property for every object you create (in my case this was for every month and row of data). Then from the matrix of object properties, I could choose those I wanted in my legend. In my case it was important to set the display year correctly as the sequence of years differed by month, but I understand a lot more about using handles now! Thanks to everyone for all their ideas, I really wasn't sure if this was solveable!!

Here's the code and graph in the end:

%Subplots
st_colours=[0, 0.4470, 0.7410;0.8500, 0.3250, 0.0980; 0.9290, 0.6940, 0.1250; 0.4940, 0.1840, 0.5560; 0.4660, 0.6740, 0.1880; 0.3010, 0.7450, 0.9330; 0.6350, 0.0780, 0.1840];
QQG_years={'2011','2012','2013','2014','2015','2016'};
figure15 = figure();
sc=gobjects(12,6);
for m=1:12
    subplot(4,3,m)
    if m==1 || m==2 || m==3 || m==4  || m==5 || m==6 || m==8 || m==11  || m==12  %Month
        for r=1:5 %The row this time
        sc(m,r)=scatter(QQG_Amm3M_T_split(m).months.NOAA_ONI(r),QQG_Amm3M_T_split(m).months.mean_Tmelt(r),25,'filled');hold on;
        end
    end
    if m==7 || m==9 || m==10
        for r=1:4 %The row this time
        sc(m,r)=scatter(QQG_Amm3M_T_split(m).months.NOAA_ONI(r),QQG_Amm3M_T_split(m).months.mean_Tmelt(r),25,'filled');hold on;
        end
    end
    if m==11 || m==12 
        for r=1:5 %The row this time
        sc(m,r).MarkerFaceColor=st_colours(r,:);
        sc(m,r).DisplayName=QQG_years{r};
        end
    elseif m==1 || m==2 || m==3 || m==4  || m==5 || m==6 || m==8
        for r=1:5 %The row this time
        sc(m,r).MarkerFaceColor=st_colours(r+1,:);
        sc(m,r).DisplayName=QQG_years{r+1};
        end
    else %m==7 or 9 or 10 
        for r=1:4 %The row this time
        sc(m,r).MarkerFaceColor=st_colours(r+1,:);
        sc(m,r).DisplayName=QQG_years{r+1};
        end
    end
    title(Month_labels(m));
    xlim([-2 2.5]);
    ylim([0 1]);
end
f1L=legend(sc([11 23 35 47 59 50]),'Location','SouthOutside','Orientation','Horizontal');
f1P = [0.5 0.02 0.03 0.03];
set(f1L,'Position', f1P,'Units', 'normalized');
%Oh look you can add a subplot title
sgtitle('Quisoquipina Glacier');

[LIU]

Thank you very much, this helps me a lot.

Answer 2

[Mario Malic]

Legend command is in the loop, therefore it is generated for each subplot. If you want to have it on particular month, you can put this within condition

if m == 1 % Jan
    legend(string(QQG_Amm3M_T_split(m).months.mean_Year));
end

Or you can put it just outside of loop, and legend will be on December.

[Catriona]

Hi Mario, thank you for our thoughts! However I only put the legend within the loop so you could see all the legends and the fact that none cover the full range of years. I don't know how to create a legend that will include all the years from all the subplots (from 2011 to 2016). Sorry for any confusion!

[Walter Roberson]

There is a legend for each axes (and subplots are axes). There is no direct method for creating an overall legend for a collection of axes.

See https://www.mathworks.com/matlabcentral/answers/542096-adding-a-global-legend-to-a-tiledlayout#answer_446195

[Catriona]

Thanks Walter, I rather feared there was not a way of creating an overall legend. I will have a look at that method you posted about using a fake legend and see if I can get it to work for my purposes.

[Mario Malic]

Having an additional scatter command for the January, you can have enough entries in the legend, and for it make the legend, but it's not a proper solution.

scatter(nan, nan);

Or maybe you could add an entry to your structure (nan, nan) to December and adjust the code, as it's easier to append the last value rather than the first.

[Catriona]

Thanks Mario, that's a nice idea, that might be quite simple to implement.