Each curve has a handle, which can be obtained from the properties.
h1 = plot(rand(1,10)); % Blue line
hold on;
h2 = plot(rand(1,10),'r'); % Red line
h3 = plot(rand(1,10),'g'); % Green line
legend([h1 h3],{'hello','world'}); % Only the blue and green lines appear
% in the legend
http://www.mathworks.com/matlabcentral/answers/406-how-do-i-skip-items-in-a-legend
