Axis

1. An axis is an imaginary line through the middle of something.
2. An axis of a graph is one of the two lines on which the scales of measurement are marked.
Axis: a) a straight line about which a body or a geometric figure rotates or may be supposed to rotate
the Earth's axis
b) a straight line with respect to which a body or figure is symmetrical the axis of a cone
— called also axis of symmetry
c) a straight line that bisects at right angles a system of parallel chords of a curve and divides the curve into two symmetrical parts
d) one of the reference lines of a coordinate system.

Axis a line. usually horizontal or vertical, used as a reference on a graph.

Sources:

Англійський словник Коллінза, електронний ресурс: https://www.collinsdictionary.com/

Словник Webster's Dictionary https://www.merriam-webster.com/

Nick Brieger, Alison Pohl: Technical English Vocabulary and Grammar