Chapter 17

Straight line graphs

17.1 Introduction to graphs

A graph is a visual representation of information,
showing how one quantity varies with another related
quantity.
We often see graphs in newspapers or in business
reports, in travel brochures and government publica-
tions. For example, a graph of the share price (in pence)
over a six month period for a drinks company, Fizzy
Pops, is shown in Figure 17.1.
Generally, we see that theshare price increases to a high
of400pinJune,butdipsdowntoaround280pinAugust
before recovering slightly in September.
A graph should convey information more quickly to the
reader than if the same information was explained in
words.
When this chapter is completed you should be able to
draw up a table of values, plot co-ordinates, determine
the gradient and state the equation of a straight line
graph. Some typical practical examples are included in
which straight lines are used.

400

Fizzy Pops

350

300

(^250) Apr 07 May 07 Jun 07 Jul 07 Aug 07 Sep 07
Figure 17.1

17.2 Axes, scales and co-ordinates

Weareprobably all familiar withreading amap to locate

a town, or a local map to locate a particular street. For

example, a street map of central Portsmouth is shown in

Figure 17.2. Notice the squares drawn horizontally and

vertically on the map; this is called agridand enables

us to locate a place of interest or a particular road. Most

maps contain such a grid.

We locate places of interest on the map by stating a letter

and a number – this is called thegrid reference.

For example, on the map, the Portsmouth & Southsea

station is in square D2, King’s Theatre is in square E5,

HMS Warrior is in square A2, Gunwharf Quays is in

square B3 and High Street is in square B4.

Portsmouth & Southsea station is located by moving

horizontally along the bottom of the map until the

square labelled D is reached and then moving vertically

upwards until square 2 is met.

The letter/number, D2, is referred to asco-ordinates;

i.e., co-ordinates are used to locate the position of

DOI: 10.1016/B978-1-85617-697-2.00017-X