When the X dot is on 0, we have to add 2 to get to where the Y dot is. X (0) + 2 = 2. That is where the Y dot is.
X (2) + 2 = 4. 4 is where the Y dot is,
Once you keep adding the X dots to 2, you will get the Y dots. That is why the equation is Y = X + 2. In order to get to Y, you need to add 2 to the X dot.