Turning data into charts: choosing well and reading right
FoundationBars compare items, pies show parts of a whole, lines show trends over time; always add a title and axis labels; a vertical axis not starting at 0 creates illusions.
- Bars compare categories, pies show parts of a whole, lines show trends.
- All pie slices must total 100%.
- Every chart needs a title and axis labels to be readable.
文體小知識：數據解讀課教你把數字變成圖表，再從圖表讀出趨勢和訊息，並會破常見的誤導手法。
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1A pile of numbers hides its patterns; a chart turns numbers into shapes so comparisons and trends stand out at a glance. Choosing the right chart comes first: bar charts compare different items, pie charts show parts of a whole, and line graphs show change over time.
2For example, a class survey on favourite sports (basketball, swimming, table tennis) is clearest as a bar chart; household spending (food, transport, entertainment) suits a pie chart; and temperatures over seven days call for a line graph. The wrong chart distorts the message.
3Making a chart takes three steps: select the data range (including the headings), insert the chart and pick the type, then add a chart title and axis labels. Without a title and labels, nobody knows what each bar or colour means.
4Charts can also mislead: starting the vertical axis at 90 instead of 0 turns a tiny gap into a 'cliff', and a pie chart with more than six slices becomes unreadable. When reading charts in adverts and news, always check whether the axes have been tampered with.
1Summary 重點摘要
Bars compare items, pies show parts of a whole, lines show trends over time; always add a title and axis labels; a vertical axis not starting at 0 creates illusions.
2Vocabulary cards Vocabulary
A visual way of presenting data, making comparisons and trends easier to see.
Uses bars of different lengths to compare quantities across categories.
Uses slices to show each part as a percentage of the whole; all slices must total 100%.
Connects data points with a line to show trends over time, e.g. temperature, sales.
The horizontal and vertical scales of a chart; starting at 0 keeps it honest.
The little key explaining what each colour or pattern in the chart represents.
A share of 100 used to relate a part to the whole; each pie slice is one percentage.
The overall direction of data over time — rising, falling or fluctuating.
A chart that creates a false impression through axis tricks or poor chart choice.
3Step by step Steps
- Three charts, three jobsChoosing the right chart comes first: to compare the size of different items, use a bar chart, e.g. comparing five subject scores; to see parts of a whole, use a pie chart, e.g. a family's spending shares; to see a trend over time, use a line graph, e.g. a week of temperatures rising and falling. Before answering, ask one question: do I want to compare, see proportions, or see a trend? The three verbs map to the three charts. A common error is using a pie chart to compare subject scores — scores are not parts of a whole, so a bar chart is right.
- Bar chart in actionFor example, 39 students were asked their favourite sport: basketball 12, swimming 18, table tennis 9; drawn as a bar chart, one glance at the three bars shows swimming leads. Bar charts place the items side by side so the gaps are obvious, and they suit data with no more than six or seven categories. When drawing, keep every bar equally wide and equally spaced, and start the vertical axis at 0 — otherwise the visual gap lies.
- Pie chart in actionFor example, monthly pocket money: food 50%, transport 20%, savings 20%, entertainment 10%; the pie shows food taking exactly half, and the proportions are instantly clear. Two rules: all the percentages must total 100, or the pie is wrong; and with too many items (more than six) the slices become too thin to tell apart, so merge the small ones into 'other' first.
- Line graph in actionFor example, a science lab measured cooling water, Monday to Friday: 80, 72, 65, 58, 50 (°C); the line graph steps steadily downward, showing the 'steady fall' at a glance, and even suggests the water will keep cooling towards room temperature. A line graph's strength is exactly this — showing change over time and hinting at the next move; temperatures and sales both suit it. Join neighbouring points with straight lines, and keep the time gaps between points equal, or the trend will be distorted. [[CHART:line:80,72,65,58,50]]
- The three-step makeMaking a chart in a spreadsheet takes three steps. Step one: select the data range, headings included, or the legend will have no names. Step two: open the Insert menu and choose the chart type — bar, pie or line, by the nature of the data. Step three: add the chart title, e.g. 'Favourite sports of our class', and label the axes, e.g. 'students' on the vertical axis, plus a legend if needed. A chart without a title and axis labels leaves readers unable to tell what each bar or colour means — and costs marks in exams.
- Three questions when readingWhen reading someone else's chart, ask three questions: what is the title? Where does the vertical axis start? What does each colour or bar mean? Only when all three are answered have you truly read the chart. For charts in adverts and reports, go further and ask whether the maker left out data, e.g. showing only the period that flatters them. The habit of three questions — check before believing — is information literacy applied to data.
- A misleading exampleCharts can mislead: two students score 95 and 92 — with the vertical axis starting at 90, the bar-length gap is exaggerated and the pair look worlds apart; starting at 0 shows the truth. Adverts love this trick: a claim of 'doubled effectiveness' may be nothing but a chopped axis. A pie chart with too many slices, or a line graph with uneven time gaps, distorts the message the same way. Always check the axes and scales first when reading a chart — and this is a common exam question too.
4Diagrams Diagrams
5Worked example Example
Example: the tuck shop sold 20, 35, 28, 40, 15 ice creams from Monday to Friday. Which chart fits best, and which day sold the most?
- Judge the nature of the data first: sales change by day (time), and the point is the trend, so a line graph fits best; a bar chart is also acceptable if only the five days are being compared.
- Draw with the days on the horizontal axis and sales on the vertical, the scale starting at 0, then join the five points with line segments in order.
- Find the peak: 40 falls on Thursday, the best-selling day; find the low: 15 falls on Friday.
- Read the trend: the sharp drop from Thursday to Friday hints the weather may have cooled, and the tuck shop could order less on Fridays.
- Answer: The data changes over time, so a line graph fits best; Thursday sold the most at 40, Friday the fewest at 15. Put the days on the horizontal axis and sales on the vertical with the scale starting at 0; the sharp fall from Thursday to Friday hints at cooler weather — a chart records data and also supports decisions.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which chart best shows how your pocket money is divided? A. Line graph B. Pie chart C. Bar chart
Q2You recorded the temperature each day for a week and want to see the trend. Which chart?
Q3What should all the percentages in a pie chart add up to?
Q4A bar chart's vertical axis starts at 95, making two classes look far apart. What is wrong?
Q5Besides the data, which two things must every chart have?
8Recap Recap
- Bars compare categories, pies show parts of a whole, lines show trends.
- All pie slices must total 100%.
- Every chart needs a title and axis labels to be readable.
- A vertical axis not starting at 0 exaggerates gaps — a common trick.
- Three reading questions: what title, where the axis starts, what each colour means.