"The first item doesn't belong to any graph" immediately indicates to me that this text does not belong in the legend, for two reasons:
- Theoretically, it does not make sense from a "grammar of graphics" approach.
- Pragmatically, whoever looks at the graphic and legend expects everything in the legend to be linked to a data element, so the viewer is going to be confused (if only briefly)
So, it is no surprise to me that it would be difficult to make this work in MATLAB. (I don't know of a way to do it.)
I would think of this text as an annotation to the legend, and would therefore would create it separately, but nearby the legend. Here is one possible way to do that. (I just made up some nonsense data.)
rng default
figure
plot(rand(6))
legend({'Spulenstrom: 0,25 A','Fit:','Spulenstrom: 0,50 A','Fit','Spulenstrom: 1,50 A','Fit'},'Interpreter','latex','FontSize',16)
annotation('textbox',[0.57,0.5,0.1,0.1],'String', ...
'Fit: $\arccos\left(\frac{\omega_0^2 - \omega^2}{\sqrt{(2\,\delta\,\omega)^2 + (\omega_0^2 - \omega^2)^2}}\right)$', ...
'BackgroundColor','white','LineStyle','-','Fontsize',12,'Interpreter','latex')
