To gain complete control over your legending use the plot-handles that plot returns. Something like this:
figure(13)
ph1 = plot(T{1:156,5},T{1:156,7},'m',T{157:308,5},T{157:308,7},'m');
hold on
ph2 = plot(T{1:156,5},T{1:156,12},'c',T{157:308,5},T{157:308,12},'c');
legend([ph1(1),ph2(1)],'Original cortisol','Smooth cortisol')
When you have the handles to the plotted lines you can include and exclude curves you want to highlight in the legend.
HTH
