Matlab is going to plot whatever you pass it. So e.g. if you want your plot only to exist between -2 and 2, then you should pass the data accordingly.
Mini example:
x = -1:0.1:1; % -1 < x < 1
y_long = sin(x);
x_short = x(x>=0); % we only want x >= 0
y_short = sin(x_short);
figure()
hold on
plot(x,y); % here, x goes from -1 to 1
plot(x_short,y_short); % here, x goes only from 0 to 1
PS: I doubt that you need this, but for completeness:
If you already have a plot, you could extract the data through get(plot_handle,'XData') etc., adjust it as desired and then set it again through set(plot_handle,'XData') etc. Don't forget to run drawnow() afterwards.
