`x<y` is not a line. One definition of a line is "a locus of point such that the x/y coordinates of each three points are pairwise proportional". Since (X,Y) with (0, 1), (0, 2) and (1, 2) all satisfy X<Y you can see that this is not a line.
You may be interested in plotting an area of X<Y. In this case plot X=Y, a 45-degree northeast-southwest line that touches the origin of the grid. And then shade in the area above (or "to the left") of the line.