Option 1
Turn off auto-updating so objects plotted after calling legend() do not appear on the legend.
lgd = legend( 'theta 30,FOV 50','theta 40,FOV 20','theta 40,FOV 30','with less error','ideal case', ...
'AutoUpdate','off');
Option 2
Plot the legend at the end and specify the object handles that should appear in the legend.
h = plot(x,y);
lgd = legend(h, 'theta 30,FOV 50');
% h can be a vector corresponding to each label name.
If you use the DisplayName property, you just need to provide the handles.
h = plot(x,y,'DisplayName', 'theta 30,FOV 50');
lgd = legend(h);
