Which chart should I use?
Start with the question the chart has to answer. The shape follows from the question, not from the data's size or how it looks in the spreadsheet.
"Which is bigger?" Bars
Separate things compared by amount: sales by product, customers by city, scores by student. One bar each, axis from zero, the biggest one highlighted. Up to about 40 bars read fine; past that, group or rank.
If the labels are long (survey answers, product names), turn the bars sideways. That's the ranking chart. Sort the rows largest to smallest first; the eye reads a sorted list in one pass.
"How did it change?" Line
Values in order: days, weeks, months, years. A line joins them so the direction shows. Two or three lines can share a chart if they use the same unit; past that they tangle.
An area chart is a line with the space under it filled. It suits a total that builds (subscribers, savings) and one series only. With negative values it looks wrong.
"What share is each part?" Donut or pie
Only when the parts add up to a whole: a budget, a vote, how a day was spent. Six parts or fewer; put the small ones together as "Other". If the parts don't add to a meaningful total (sales by month, say), a pie is wrong: use bars.
"How does a total split?" Stacked bars
Several totals, each made of the same parts: revenue by channel per quarter. The stack height is the total and the segments are the parts. Reading the middle segments is hard, so if the parts matter more than the totals, use grouped bars (several series, one bar each).
"Do these two go together?" Scatter
Two measurements for each row: hours studied and score, temperature and ice-cream sales. One dot per row. The explanation panel gives the correlation, r, between −1 and 1. Near 0 means no straight-line relationship. A strong relationship still doesn't say which one causes the other.
Four habits that make charts lie
- An axis that doesn't start at zero on a bar chart. The bar's length is the value; cut the axis and a 3% difference looks like 300%. ChartCamel always starts bars at zero.
- A line through categories. A line says "these are in order". Students, products and cities aren't.
- Too many pie slices. Past six, nobody can compare them. The explanation panel warns you.
- Percentages of a partial total. A donut shows each part's share of what you pasted. Leave out a part and every other percentage is too high.