A line or ray pointing straight upward will be pointing in the direction of maximum increase. It will make an angle of 90° with respect to the x-axis.
A line or ray pointing straight downward will be pointing in the direction of maximum decrease. It will make an angle of -90° with respect to the x-axis.
A horizontal line will be associated with zero change. It makes an angle of 0° with respect to the x-axis.
