Underscore doesn't work for step legend

I'm trying to plot two step responses alongside with experimental results. Plot looks okay, but for some reason Matlab inserts a backslash before underscore for the step legend. How do I fix the legend?

\omega_{experimental} and \theta_{experimental} work

\omega_{modelo} and \theta_{modelo} don't

f2 = figure('DefaultAxesFontSize', 14);

plot(t_exp,omega_exp,'red');
hold on
plot(t_exp,theta_exp,'blue');
hold on
step(12*Go);
hold on
step(12*Gt);

title('Comparação do modelo obtido com a resposta experimental')
xlabel('Tempo (segundos)')
ylabel(' ')
legend({'$\omega_{experimental} (rad/s)$','$\theta_{experimental} (rad)$',...
    '$\omega_{modelo} (rad/s)$','$\theta_{modelo} (rad)$'},...
    'Interpreter','latex','Location','southeast','FontSize',16);
set(gca,'XLim',[0 1])

This is what I'm getting:

This is the backslash that Matlab inserts for some reason (seen by double-clicking the plot legend):

Full code is attached. Thanks in advance!

[Adam Danz]

Works fine for me (copied your call to legend)

figure
plot(rand(5,4))
legend({'$\omega_{experimental} (rad/s)$','$\theta_{experimental} (rad)$',...
    '$\omega_{modelo} (rad/s)$','$\theta_{modelo} (rad)$'},...
    'Interpreter','latex','Location','southeast','FontSize',16);

[Vitor Sternlicht]

Yep, it works fine with plot. Not with step though. I need it to work with these:

plot(t_exp,omega_exp,'red');  % works
hold on
plot(t_exp,theta_exp,'blue'); % works
hold on
step(12*Go);  % doesn't work
hold on
step(12*Gt);  % doesn't work

For some reason Matlab messes up the legends after steps.

Answer 1

[Adam Danz]

The escape character (\) is mistakenly added in legend() > setDisplayNames() even though the latex interpreter is correctly assigned. This evokes updateGroupInfo() which adds the escape character. Even if you assign the string to the DisplayName property prior to calling legend(), it immediately addes the escape character. However, the string is correctly assign in latex (or tex) format if the assignment is done after the legend is created. This is because you are only altering the legend string and therefore bypassing the updateGroupInfo() function.

Links to other threads in this forum that have revealed this problem with other plotting functions:

• step() (this thread)
• nichols()
• bode()

Given that, here are two work-arounds. Also see per isakson's solution below for a 3rd alternative.

Workaround 1

Call the legend twice.

% reproducible example
figure(); 
s = tf('s');
G = exp(-s) * (0.8*s^2+s+2)/(s^2+s);
T = feedback(ss(G),1);
step(T)

legend({'$\omega_{modelo} (rad/s)$'},...
    'Interpreter','latex','Location','southeast','FontSize',16);   
legend({'$\omega_{modelo} (rad/s)$'})

Workaround 2

Assign the DisplayName property after creating the legend.

figure
s = tf('s');
G = exp(-s) * (0.8*s^2+s+2)/(s^2+s);
T = feedback(ss(G),1);
step(T);

legend('Interpreter', 'latex') 
axisObjs = get(gca, 'Children'); 
set(axisObjs(1), 'DisplayName', '$\omega_{modelo} (rad/s)$')

Answer 2

[per isakson]

With R2018b. Deep inside and after too many if-statements to keep track of a backslash is added.

C:\Program Files\MATLAB\R2018b\toolbox\shared\controllib\graphics\@wavepack\@waveform\updateGroupInfo.m

line 19-21

if ~this.LegendSubsriptsEnabled
    dispname = strrep(dispname,'_','\_');
end

C:\Program Files\MATLAB\R2018b\toolbox\matlab\scribe\legend.m

Why Line and Group, respectively?

The order of the text-arguments in the call of legend() matters

And in accordance with Adam Danz replace legend(___) by

lh = legend({
        '$\omega_{experimental} (rad/s)$'   ...
    ,   '$\theta_{experimental} (rad)$'     ...
    ,   '$\omega_{modelo} (rad/s)$'         ...
    ,   '$\theta_{modelo} (rad)$'       }   ...
    ,   'Interpreter','latex','Location','southeast','FontSize',16);

lh.String{3} = '$\omega_{modelo} (rad/s)$';
lh.String{4} = '$\theta_{modelo} (rad/s)$';
lh.String{4} = '$\theta_{modelo} (rad/s)$';

String(4) twice is not a copy&paste error!

[Adam Danz]

+1

same in r2019a

[Walter Roberson]

Good tracking! I had just traced it down to that same assignment in legend.m, but had not at all noticed the listener that triggers the behaviour you noticed!