The elements of a graph

Why use a graph?

In physics, we often measure a quantity that changes as a function of another: the distance travelled by a ball over time, the temperature of water as it heats up… A graph allows us to visualise this change at a glance, whereas a table of numbers is more difficult to interpret.

Essential elements

A well-constructed graph always includes:

• a title indicating what is being represented;
• a horizontal axis (x-axis) and a vertical axis (y-axis);
• the name of the quantity and its unit on each axis (e.g. ‘time (s)’);
• a uniform scale on each axis;
• the origin, the point where the two axes intersect (often (0;0)).

Example

To plot the distance travelled by a car against time:

Quantity	Axis	Unit
Time	horizontal (x-axis)	seconds (s)
Distance	vertical (y-axis)	metres (m)

Common pitfall

Omitting the unit on an axis renders the graph unusable: ‘10’ on its own means nothing, whereas ‘10 s’ is precise. Always ensure that each axis has a label AND a unit; otherwise, the graph cannot be interpreted correctly by anyone else.