Answer
Fill the blank with : horizontal.
Work Step by Step
A function is a correspondence between a first set, called the domain,
and a second set, called the range,
such that each member of the domain corresponds to exactly one member of the range.
The graph of a function is the set containing points $(x,f(x))$,
where x is an element of the domain, and f(x) is the corresponding element of the range,
meaning that we use the horizontal axis for x, the domain,
and the vertical axis for f(x), the range.
Fill the blank with : horizontal.