Graphs and Charts
Matching the display to the data.
A chart is an argument about data. The type you choose decides which comparison the reader makes easily and which they cannot make at all.
Matching chart to question
| Question | Display |
|---|---|
| How do categories compare? | Bar chart |
| How does one series change over time? | Line chart |
| How are values distributed? | Histogram or box plot |
| Do two variables move together? | Scatter plot |
| How does a whole split into parts? | Stacked bar, rarely a pie |
Bar charts and histograms are not the same
A bar chart compares separate categories and its bars are drawn apart. A histogram shows the distribution of one continuous variable across intervals and its bars touch, because the underlying scale is unbroken. Swapping them misrepresents the data.
Choices that mislead
Truncating a bar chart's vertical axis exaggerates small differences, because a bar's length is doing the comparing. Line charts may start above zero when the question is about change rather than magnitude. Pie charts fail whenever slices are close in size, since angle is hard to judge.
Related: statistics, and the number base converter for the binary axis labels used in computing charts.