It may be difficult to attribute the following points to a specific source, but here are all of the guidelines I can remember off the top of my head about bar charts vs. line charts, mostly learned from Edward Tufte and Stephen Few. It’s a bit of an art, and how you represent your data depends on what exactly you are intending to find in it, so it’s difficult to write finite rules that dictate what to do. If you want to learn more, check out their books on our Reading page.
- When to use them: Line charts should be used only for time series (chronological) or when there is some other sequence to the dimensions on the x-axis, e.g. dates, months, sequence of stages of a project, sequence of meters along on a gas pipeline, and they should be used to detect trends and patterns, not to give people exact quantitative readings.
- Scale: As line charts are not really intended to give people exact numbers, forcing zero scaling is not necessary and can make it considerably more difficult to detect said trends and patterns.
- When to use them: Bar charts should be used for comparing specific x-axis values, though they can certainly be used for time series, like line charts. They can also be used to display parts of a whole in favor of pie charts, in which case, the space between the bars should be reduced.
- Orientation: Do not use vertical or diagonal text to label the axis of a bar chart. If the x-axis has longer text descriptions, use horizontal bar chart, so the text can read left-to-right, horizontally (the way we normally read).
- Scale: As the area of bars implies volume, it can be deceptive to use dynamic scaling with bar charts (see: Lie Factor). If the differences between the data points is difficult to distinguish with forced-zero scaling, use symbols/points in favor of bars and use dynamic scaling.
Applies to both
- Dimension order: There should be some logical order to the dimensions on the x-axis. In the case of a line chart, it should follow the chronological, process, or stage order that caused you to select a line chart in the first place. In the case of bar charts, the order should have some rhyme and reason to it: sorted by y-axis value, alphabetical, etc., depending on the content of the chart and what its intended use is, e.g. ranking, distribution.
- Scale labels: If the numbers are already being displayed on the data points, it is redundant to label the axis with numbers, too.
- Axis labels: If you can incorporate the metric names and dimension names into the chart title or legend, do not waste space on axis labels.