Draw the graph `x -y le 1` in the cartesian plane

Explanation

(a) First draw the line `x -y =1 " i.e., " y = x -1`
`{:(x,1,0,-1),(y,0,-1,-2):}`
As a first step, plot the ordered pairs (1,0), `(0, -1), (-1, -2)` and then join them with a line

(b) For shading the region we consider any point in one of the half planes.
Let us consider the point (0,0). Here, boundary line `x -y =1` is included