All real-life graphs can be used to estimate or read-off values.The actual meaning of the values will depend on the labels and units shown on each axis. Sometimes:
the gradient of the line or curve has a particular meaning
the y-intercept (where the graph crosses the vertical axis) has a particular meaning
the area has a particular meaning
Example:
This graph shows the cost of petrol.
It shows that 20 litres will cost £23 or £15 will buy 13 litres.
\(gradient = \frac{change~up}{change~right}\) or \(\frac{change~in~y}{change~in~x}\)
Using the points (0, 0) and (20, 23), the gradient = \(\frac{23}{20}\) = 1.15.
The units of the axes help give the gradient a meaning.
The calculation was \(\frac{change~in~y}{change~in~x} = \frac{change~in~cost}{change~in~litres} = \frac{change~in~£}{change~in~l} = £/l.\)
The gradient shows the cost per litre. Petrol costs £1.15 per litre.
The graph crosses the vertical axis at (0, 0). This is known as the intercept.
It shows that if you buy 0 litres, it will cost £0.
Example:
This graph shows the cost of hiring a ladder for various numbers of days.
Using the points (1, 10) and (9, 34), the gradient \(= \frac{change~up}{change~right}\) or \(\frac{change~in~y}{change~in~x} = \frac{34-10}{9-1} = \frac{24}{8} = 3\).
The units of the axes help give the gradient a meaning.
The calculation was \(\frac{change~in~y}{change~in~x} = \frac{change~in~cost}{change~in~days} = \frac{change~in~£}{change~in~days} = £/day.\)
The gradient shows the cost per day. It costs £3 per day to hire the ladder.
The graph crosses the vertical axis at (0, 7).
There is an additional cost of £7 on top of the £3 per day (this might be a delivery charge for example).