Signs

WHEN WE LOOK AT THE 2D

1. Let name the horizontal line as X
2. Let name the vertical line as Y
3. As X intersect Y, it is the origin point

FOR REFERENCE 

1. Moving right from the origin point is the positive sign
2. Moving to the left from the origin point is the negative sign
3. For the Y, moving up from the origin is positive
4. Moving down from the origin is the negative

We can name X as something else and Y as something else.  For example;  we can say the X as the supply line and the Y as Demand.