Use bars for categories, lines for ordered change, and pie sectors for parts of a defined whole.
Use a bar graph to compare categories. For the practice counts walking 12, bus 6 and car 2, draw equal-width bars with gaps. Label the transport categories and use a regular count scale starting at zero. Add a title and units. Do not join the categories with a line: walking, bus and car are not points in a continuous sequence.
Use a line graph for change along an ordered scale, such as time. The visitor counts at 09:00, 10:00, 11:00 and 12:00 are 20, 35, 30 and 50. Put evenly spaced hours on the horizontal axis and counts on the vertical axis. Plot each pair, then join them in time order.
Describe both the overall trend and the exception. Counts rise from 20 to 50, an increase of 30 visitors. They also fall from 35 to 30 between 10:00 and 11:00, so the rise is not continuous.
To compare two groups, use separate bars or lines and a clear key. Keep the time intervals and measurement methods comparable. Otherwise, a difference may come from how the data were collected.
A pie chart shows parts of one total. Of 20 travellers, walking accounts for 12, or 60%; bus for 6, or 30%; and car for 2, or 10%. Each sector angle is count / total x 360 degrees. The angles are 216, 108 and 36 degrees. One pie chart shows shares, but cannot show growth without comparable earlier data.
Step by step
Choose the graph
Use bars for categories, a line for change over time, and a pie for shares of one total.
Draw it clearly
Add a title, labels, units and a regular scale. Add a key when comparing groups.
Check and describe
Check each plotted value. State the overall pattern, support it with figures and include an important exception.
Worked example: Compare the same hours
Suppose a second practice morning has 40 visitors at 09:00 and 45 at 12:00. It begins 20 higher than the first morning, but ends 5 lower. This crossover is more informative than saying one morning is always busier. Keep the same observation duration before comparing counts.
Watch out for this
A higher final point proves the line increased at every interval.
An overall increase can contain falls. Inspect each interval and mention important exceptions.
Check your understanding
Which description matches the values 20, 35, 30, 50 at consecutive hours?
- An overall rise, interrupted by a five-visitor fall between the second and third hours.
- A continuous increase of exactly ten visitors every hour.
- No pattern can be described because one value falls.