Bar Charts
A bar chart uses rectangular bars to show the frequency (count) or value of different categories. The height (or length) of each bar represents the value for that category.
Key features of a bar chart:
- A title explaining what the chart shows
- Labelled axes (horizontal = categories, vertical = frequency/value)
- Equally spaced bars, usually with gaps between them
- A scale on the vertical axis
Bar charts are excellent for comparing different categories — for example, the number of students who prefer different school subjects.
Reading a Bar Chart
Example: A bar chart shows ice cream sales for one week.
| Day | Sales |
|---|---|
| Monday | 20 |
| Tuesday | 35 |
| Wednesday | 15 |
| Thursday | 40 |
| Friday | 50 |
- Highest sales: Friday ()
- Lowest sales: Wednesday ()
- Total sales:
Read bar charts carefully. Check the scale — each square on the grid might represent 2, 5, or 10 units, not just 1!
Line Graphs
A line graph connects data points with a line and is best for showing how something changes over time.
Example: A line graph shows a plant's height each week:
| Week | Height (cm) |
|---|---|
| 1 | 3 |
| 2 | 7 |
| 3 | 12 |
| 4 | 18 |
| 5 | 22 |
The rising line shows the plant is growing steadily. A steep section means fast growth; a flat section means little growth.
Drawing a Bar Chart — Steps
- Choose a suitable scale for the vertical axis.
- Draw and label both axes.
- Draw a bar for each category with the correct height.
- Give the chart a title.
Do not start your vertical axis at a value other than zero unless you have a good reason — it can make small differences look much bigger than they really are.
Real world: school canteen choices
A canteen manager records students' lunch choices for one week: Monday 80, Tuesday 95, Wednesday 70, Thursday 110, Friday 105. A bar chart instantly shows Thursday and Friday are the busiest days, helping staff schedule extra cooks on those days.
Real world: a shop's monthly sales trend
A bookshop plots monthly sales on a line graph. Sales rise steeply from September to December (back-to-school and Christmas), then drop sharply in January. The steeper the line, the faster sales are changing — line graphs make these trends easy to communicate to investors.
Practice Problems
A bar chart shows: Cats = 8, Dogs = 14, Fish = 6, Birds = 4. How many more people have dogs than cats?
Show answer
From the ice cream data above, what is the mean (average) daily sales?