Very nice explanation, Aziz...!!!
Here is another way to look at this, analytically (if not particularly "mathematically")...Note that, when lyl =0, lxl cannot be larger than 1. Thus, x cannot be more than +1 or less than -1. So, plot the two points on the graph (1,0) and (-1,0). Likewise, when lxl = 0, lyl cannot be more than 1 or less than -1. Then, also plot the two points (0,1) and (0, -1) on the graph. Then, let all four points be the vertices of a square. The sides of this square are the "bounds" of our graph. Note that every point lying on any of the four sides satisfies lxl + lyl = 1, and any point lying "outside" these bounds, doesn't. (They make the equation > 1).
This gives us a way to graph any equation of the form lxl + lyl = a....where a is some positive real number.
Does that help??