See if this does what you want:
[hLeg,hObj]=legend([plot2 plot1 plot3 plot4 ],{'experimental data d=3.5','Model 1: d=3.5','Model 2: d=3.5','Model 3: d=3.5'})
set(legend,'Location','NorthWest')
set(legend,'FontSize',12)
axis([5,15,6,7.8])
set(gca,'fontsize',12)
xlabel('Wall thickness [mm]') % x-axis label
ylabel('ln(F_{max,k}) [-]') % y-axis label
legend('boxoff') % <= Change This Line
I can’t run your code, but that should work (in R2016a). If you have a different version, the command to turn the legend box off may be different.
