For graphic objects, there is a 'Visible' Property that you can adjust.
If you have a handle for your line object, for example:
a = plot(1:5,1:5); % a is your handle to the line
set(a,'Visible','off'); %'off' or 'on'
%or
a.Visible = 'off';
if you wish to do this beyond the scope of the original function, you can look for this line object by accessing the children of the axes.
g = gca; %gets the current axes
objs = g.Children %this will list all line objects in your g axes
%then from here you can index to the appropriate line object you want
