Bar chart (II): Comparison

1Summary 重點摘要

Compare two sets side by side.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Part 1: What a double bar chart looks likeA double bar chart compares two sets of data. To compare the sports Class A and Class B like best, every sport on the horizontal axis has two bars side by side: one for Class A, one for Class B, drawn touching each other. The side-by-side design lets the two groups be compared directly within the same category, without flipping between two separate charts — quick and accurate.
2. Part 2: The legend is the keyThe legend, usually in a corner, uses colour squares and words: blue means Class A and orange means Class B. The legend is the key to the whole chart — without it nobody can tell which bar belongs to which class, and no comparison is possible. So the first step in reading is always to check the legend; when drawing your own chart, never forget to add one, or readers will mix up the two sets.
3. Part 3: Check the scale, then readBefore reading any bar, confirm the vertical scale: what each grid line is worth. If each line means 5 people, then in the football pair the blue bar reaches line 2, so 2 × 5 = 10 people, while the orange bar reaches line 3, so 3 × 5 = 15 people. Mistake the grid for 1 and every reading is only a fifth of the truth. The safe order is: check the scale, count the lines, then multiply.
4. Part 4: Compare side by sideComparing the two bars within one category gives the gap between the classes. In football the orange bar is taller, so Class B has more fans: 15 − 10 = 5 people. The height difference between the two bar tops equals the difference between the two groups, so differences are found by subtraction: larger minus smaller. The design shows the gap at a glance, but written answers still need the subtraction and the unit.
5. Part 5: Biggest gap and totalsWork out each gap in turn and the biggest one stands out: football differs by 5, basketball by 10, while swimming is 9 in both classes — a difference of 0 — so basketball shows the largest contrast. If the question asks for a total instead, such as how many in both classes like football, switch to addition: 10 + 15 = 25. Always check whether the question wants a difference (subtract) or a total (add).
6. Part 6: Drawing one wellTo draw your own double bar chart: first list the categories on the horizontal axis; second choose a vertical scale that still fits the largest value; third draw two equal-width, touching bars for every category at the correct heights; fourth colour the two sets differently; fifth write the legend in a corner and add a title. Without a legend or with an unclear scale the chart loses its value, so check every part after drawing.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• A double bar chart pairs two bars per category to compare sets
• The legend names each colour's set — it is essential
• Read the scale first: lines × scale
• Difference: subtract; total: add
• Compare only within the same category