The answer depends on how you're storing your coordinate data. Is it stored as two columns, the first x and the second y?
If so just add your constant to the second column of xy.
Is your data stored as arrays of points each containing x, y, and z data?
If so add your constant to the y array.
Is your data stored as an image?
im = imread('ngc6543a.jpg');
In this case you'll probably want to add the 'XData' and 'YData' options to your imshow call.
Or is it stored in some other way? If so please describe how you're storing your data.