Do you just want to offset the data once or do you want to move it after you already plotted it?
For offsetting just once, just add the offset to your y vector.
plot(x1,y1 + offset,'r-')
If you want to move an existing line you can store the handle when you plot it the first time and then modifiy it with the set command.
% plot and store handle
handleOfLine = plot(x1,y1,'r-');
% change data
set(handleOfLine,'YData',y1 + offset);
% refresh the figure
drawnow
Place this into a loop and add a pause() call somewhere and you can create a moving plot.
