Start with the question
A chart is a way of answering a question quickly. Before opening any software, write the question in one sentence. For example: “Which of my four modules took the most study time this term?” or “How did my weekly practice scores change over eight weeks?” The question tells you which kind of chart is likely to work, and it tells you what the title should say.
If you cannot state the point of a chart, readers will struggle to find it as well. A table of numbers is sometimes the better choice, particularly when the exact values matter more than the pattern.
Choosing between bar, line and pie
Bar charts compare amounts across separate categories: hours per module, number of responses per option, scores for different groups. The length of each bar is easy to judge by eye, which is why bar charts are a safe default. Keep the bars in a meaningful order, such as largest to smallest, unless the categories have a natural order of their own.
Line charts show how something changes over a continuous sequence, usually time. The line connects points in order, which suggests a trend. Use one when the horizontal axis is time (days, weeks, years) and the order matters. Do not use a line to join unrelated categories, because the slope between them would mean nothing.
Pie charts show parts of a whole, and only work well with a small number of slices, perhaps up to four or five, where the differences are large. People find it hard to compare angles and areas accurately. If you have many slices, or slices of similar size, a bar chart is usually clearer. If the parts do not add up to a meaningful whole, do not use a pie at all.
A short summary:
| Question | Usually suits |
|---|---|
| Which category is biggest? | Bar chart |
| How has this changed over time? | Line chart |
| What share does each part take of the total? | Pie chart, with few slices |
| What are the exact values? | A table |
These are guidelines, not laws. Other chart types exist, such as scatter plots for the relationship between two measurements, but the three above cover most everyday needs.
Label so the reader does not have to guess
A chart should make sense without the surrounding text. At a minimum:
- A specific title that says what is shown, such as “Weekly study hours by module, fall term”, not just “Study hours”.
- Axis labels with units, such as “Hours per week” or “Score out of 20”.
- Readable category names, shortened if needed but not cryptic.
- A legend only if there is more than one series; otherwise label directly.
- A source and date for data you did not collect yourself, and a note on sample size if relevant.
Value labels on bars can help when there are only a few of them. Too many numbers on a crowded chart, though, make it harder to read.
Be honest about the axes
Many misleading charts are misleading because of axis choices rather than outright errors.
Bars should start at zero. The length of a bar represents the amount, so a bar chart whose axis begins at, say, 50 exaggerates differences. If the interesting variation sits far from zero, a line chart or a dot chart can be fairer, with the axis clearly labeled.
Line charts can use a non-zero axis, but it should be visible and noted, because a stretched vertical scale can turn a small change into a dramatic-looking one.
Avoid decoration that distorts. Three-dimensional effects, tilted pies and decorative pictures make values harder to read accurately. Plain is better.
A quick check: imagine someone who only looks at the picture for five seconds. Would they come away with an accurate impression? If not, change the chart. Learning to spot misleading charts in other people’s work will sharpen your own.
A small worked example
Suppose, hypothetically, you track hours spent on four modules in a week: Math 6, Writing 4, History 3, Biology 2. The total is 15 hours. Which chart fits?
For the question “which module takes the most time?”, a bar chart sorted from Math down to Biology works well. Math at 6 hours is clearly the longest bar.
For “what share of my time goes to each?”, the shares are 6 of 15 (40%), 4 of 15 (about 27%), 3 of 15 (20%) and 2 of 15 (about 13%). Together these come to 100% once rounding is allowed for. A pie with four slices could work, though the bar chart still makes the comparison easier. If you want to see how these hours changed across the term, switch to a line chart with weeks along the bottom.
If you need to refresh how shares are worked out, a refresher on percentage calculations will help, and being able to do graph reading confidently makes it easier to check your own work.
Color and accessibility
Use color sparingly. Many people have color vision differences, and charts get printed in greyscale, so add direct labels or patterns. Add a short text description for readers using assistive technology.
Check before you share
- Does the title say what the chart shows?
- Do the axes have labels and units?
- Does a bar chart start at zero?
- Is the order of categories sensible?
- Is every number correct against the original data?
- Would the chart still make sense without color?
- Is the source named?
Making a chart in a spreadsheet takes a few clicks, but checking its honesty takes a few minutes more, and that is where most of the value lies. Charts show what the data says, and they cannot show why. When you draw conclusions, be careful about claiming more than the data supports.