What is J? I'll assume it's a 1 by 2 array with the x (column) coordinate, followed by the y (row) coordinate. Then to plot 300 columns to the right, do
plot(J(1)+300, J(2), 'r*', 'MarkerSize', 20, 'LineWidth', 2);
Make sure you're not making the very common mistake of mixing up x,y with row, column - that's almost certainly what you're doing. Remember images take indexes of (row, column) which is (y, x) while plot() takes (x,y) which is (column, row). Make sure you get it straight - like I said, this is a very common error.