How do I create a legend that explaines scatter plot by marker size

I am creating a bubble chart (scatter plot with different marker sizes). This works fine but I would need a legend to show the range of the parameter that defines the size of the marker.

The code for the plot looks like this:

scatter(r1,r2,a);

a is an array, that determines the size of the markers.

So I am looking for a legend like the one I attached.

Appreciate any help, thanks a lot!

[Ali]

if true
--------------------------------------------------- code start
This is an example for your case Nelly
Input is "Input_Data", two dimension matrix 
Marker_Counter=1;
   figure6=figure;
   Markers = {'+','o','*','x','v','d','^','s','>','<'};
   for i=1:10:size(Input_Data,1)
    TPR=Input_Data(i:i+9,7);
    FPR=Input_Data(i:i+9,8);
plot(FPR,TPR,strcat('-',Markers{Marker_Counter}));
Marker_Counter=Marker_Counter+1;   
hold on
   end
plot([0.5 1],[0.5 1],'--');
legend('Minpts = 100','Minpts = 200','Minpts = 300','Minpts = 400','Minpts = 500','Minpts = 600','Minpts = 700','Minpts = 800','Minpts = 900','Minpts = 1000','','Location','SouthEast');
xlabel('FPR or (1-Specificity)','FontSize',12,'FontWeight','bold'); ylabel('TPR or Spensitivity)','FontSize',12,'FontWeight','bold');
title('ROC Space');
close(gcf);
-------------------------------------------- code end
end
--------------------------------------- picture link preview
<</matlabcentral/answers/uploaded_files/92608/untitled.bmp>>

Answer 1

[Joseph Cheng]

since you're using scatter one way around this is by doing this:

r1 = randi(10,1,10);
r2 = randi(10,1,10);
a = 30*randi(5,1,10);

bubsizes = unique(a)';
legentry=cell(size(bubsizes));
figure,hold on
for ind = 1:numel(bubsizes)
   bubleg(ind) = plot(0,0,'ro','markersize',sqrt(bubsizes(ind)),'MarkerFaceColor','red');
   set(bubleg(ind),'visible','off')
   legentry{ind} = num2str(bubsizes(ind));
end

h = scatter(r1,r2,a,'r','MarkerFaceColor','red')
legend(legentry)

I couldn't find a quick solution to the scatter plot marker area to plot's "markersize" but the sqrt() of the scatterplot marker area visually looks to be about the same.

[Andreas Kaineder]

great!

I adapted your code a bit to only show 5 sizes in the legend by:

bubsizes = [min(a) quantile(a,[0.25, 0.5, 0.75]) max(a)];

thanks a lot for your help!

[Joseph Cheng]

i would hope you did, i just did that to create dummy data.

Answer 2

[Mike Garrity]

You need a separate object for each legend entry. Something like this:

rng default
x = randn(1,100);
y = randn(1,100);
a = randi(5,[1 100]);
plot(x(a==1),y(a==1),'o','MarkerSize',5 , ...
     'MarkerFaceColor','red','MarkerEdgeColor','black')
hold on
plot(x(a==2),y(a==2),'o','MarkerSize',10, ...
     'MarkerFaceColor','red','MarkerEdgeColor','black')
plot(x(a==3),y(a==3),'o','MarkerSize',15, ...
     'MarkerFaceColor','red','MarkerEdgeColor','black')
plot(x(a==4),y(a==4),'o','MarkerSize',20, ...
     'MarkerFaceColor','red','MarkerEdgeColor','black')
plot(x(a==5),y(a==5),'o','MarkerSize',25, ...
     'MarkerFaceColor','red','MarkerEdgeColor','black')
legend show

[Andreas Kaineder]

thanks a lot for your input! unfortunately I can't separate the plots like you did, because I have hundreds of different values in the array a. the legend only needs to show the size of the minimum, q1, q2, q3 and the maximum of the data.

seems to be tricky...

[Mike Garrity]

In that case I would create invisible scatter objects to stand in for each of the "size buckets" you want. Set those Visible='off' and leave the "real" scatter out of the legend.

[vadim girardeau]

Hello Mike,

In matlab 2023b it does not work anymore, the size is like autoupdate for the legend, do you know how to fix it ? Thanks a lot :

Answer 3

[dpb]

"Use the handles, Luke..." :)

If you save the handles to the scatter points, you can write a legend for those specific points using the handles and the desired text (in your example that will be the num2str output for the given value in the format desired).

The default legend marker is the same size for each; if you save the object handles (second optional output) from legend, there are two sets of them, a text and a patch object. The first N are the text; the second set of N are the patches. Set the 'markersize' argument for those to match the size desired.

It looks like you may have to futz with the spacing within the legend box as well; it seems to not reflect a size based on the marker size.

Lotta' work, but doable...there's a lot of flexibility available in HG but there's a lot of stuff that just seems excessively needing of user futzing, too, unfortunately.

[dpb]

"t looks like you may have to futz with the spacing within the legend box as well; it seems to not reflect a size based on the marker size."

This would seem worth an official enhancement request filing to TMW at the www.mathworks.com site; a legend should reflect the markers and be "smart" enough to not put them on top of each other as Mike's plot illustrates (and is the end result with doing it directly on the original legend handles as I suggested, too; I just didn't attach the sample figure but the end result is identical).