It is not necessary that the function decreasing over a given interval always be negative.
A function f(x) (value) decreases as x increases.
This does not mean that value of f(x) is negative.
It can have positive number as range.
12 cm:102 ft
-------- ------
12 12
1 cm:8.5 ft
The scale is 1 cm equals 8.5 ft.