All you need to do is find coordinates of 2 points and connect them with straightedge and extend them
Y = x
X= 0 y =0 (0,0) is one point.
X=1 Y=1 ( 1,1) is 2nd point
Connect the line segment passing through origin (0,0) and (1,1) and extend.
When x=y, whatever you plug for x it equals y, so plot on x/y would be data points like this (0,0), (1,1), (2,2), etc. same goes for negative numbers (-1, -1), what you have is a diagonal line through quadrants II and III going through (0,0).