How to manage the legend for many plots - matlab

Question

How do I handle this issue
clear all; close all; clc;
r = 0:5;
zeta = 0:0.1:1;
for i = 1:size(zeta(:))
for j = 1:size(r(:))
X(i,j) = sqrt((1+(2*zeta(i)*r(j))^2)/((1-r(j)^2)^2+ (2*zeta(i)*r(j))^2));
end
plot(r,X);
xlabel('r = \omega/\omega_n');
ylabel('M = \frac{X}{Y}');
hold all
grid
[~,~,~,current_entries] = legend;
legend([current_entries {sprintf('\zeta = %i',zeta(i))}]);
end
figure
plot(r,X)
grid
The hold all command doesn't seem to be working properly. What can I do to fix this?

Answer 1

You will want to set the DisplayName property of the plot. Then when you create the legend, the labels will automatically be populated.
plot(r, X, 'DisplayName', 'name')
Also, the string that you were passing to sprintf for your legends needs to be escaped becuase sprintf thinks that \z is a control character.
plot(r, X, 'DisplayName', sprintf('\\zeta = %0.1f',zeta(k)))
Also, hold on is recommended over hold all. Also, it is best practice to specify the axes handle when calling hold to ensure that it is applied to the current axes.
hold(hax, 'on')
So if we incorporate these changes into your plotting code (along with #R.Falque's idea to use semilogy)
r = 0:0.001:5;
zeta = 0:0.1:1;
hax = axes();
colors = hsv(numel(zeta));
for k = 1:numel(zeta)
X = sqrt((1 + (2 * zeta(k) * r).^2) ./ ((1-r.^2).^2+ (2*zeta(k)*r).^2));
semilogy(r, X, ...
'DisplayName', sprintf('\\zeta = %0.1f',zeta(k)), ...
'Color', colors(k,:));
hold(hax, 'on')
end
grid(hax, 'on')
xlabel(hax, 'r = \omega/\omega_n', 'Interpreter', 'tex');
ylabel(hax, 'M = $\displaystyle\frac{X}{Y}$', 'Interpreter', 'latex');
L = legend('show');

Answer 2

You can also use cells array as follow:
clear all; close all; clc;
r = 0:0.001:5;
zeta = 0:0.1:1;
figure;
for i = 1:length(zeta)
for j = 1:length(r)
X(j) = sqrt((1+(2*zeta(i)*r(j))^2)/((1-r(j)^2)^2+ (2*zeta(i)*r(j))^2));
end
semilogy(r,X);
hold on
legend_string{i} = ['\zeta = ', num2str(zeta(i))];
end
hold off
grid
xlabel('r = \omega/\omega_n');
ylabel('M = $\frac{X}{Y}$','Interpreter', 'Latex');
legend(legend_string);
Note that you have an error in the X definition (corrected from X(i,j) to X(j)).