With positive slope the line moves upward when going from left to right ... This is a special case of a horizontal demand curve which says at any price above P* demand drops to zero. An example might ...

The slope of a line can be 'seen' to be sloping uphill ... gradient\,of\,line\,EF = - \frac{2}{5}\) Horizontal lines have a gradient that is zero, ie they are not sloping uphill or downhill.

The National 4 section shows how to calculate the gradient of a slope using the vertical height and horizontal distance. A similar approach can be used for calculating the gradient of a line ...