Hi Milad,
You can create legends with subscripts by using the LaTeX interpreter. Use the underscore character _ to denote subscripts, and set the interpreter to 'latex'.
Attaching a small code for reference:
x = linspace(0, 2*pi, 100);
y1 = sin(x);
y2 = cos(x);
plot(x, y1, x, y2);
% Create a legend with subscripts
legend({'$y_1 = \sin(x)$', '$y_2 = \cos(x)$'}, 'Interpreter', 'latex');
You can also refer to the documentation Add legend to axes - MATLAB legend (mathworks.com) for further reference.
