Line chart vs area chart
An area chart is a line chart with the space beneath it filled. The fill is not decoration: it implies that the area is a quantity. Use it when that is true and it helps; skip it when it is not, because the ink adds weight without adding meaning.
Both charts below are the same data — Cumulative revenue — so the difference you see is the chart type, not the numbers.
Line chart
Use it when
- The metric is a rate, an index or a price — the area under it is meaningless.
- You are plotting more than one series and the fills would occlude each other.
- The value does not start at zero.
- Direction and slope are the point, not magnitude.
Area chart
Use it when
- The quantity accumulates: revenue to date, energy consumed, storage used.
- There is a single series and volume is the message.
- The baseline is a true zero.
- You want the chart to read as 'how much' rather than 'which way'.
Which one
One series that starts at zero and represents a volume: fill it. Anything else, and especially anything with two or more series or a percentage on the axis: leave it as a line.
Questions people ask
- Can I use an area chart for percentages?
- Only for a stacked area totalling 100%, where the bands are shares of a whole. A single percentage series filled to zero implies the area under a rate is a quantity, which it is not.
- How many series can an area chart hold?
- One, unless you stack them. Two overlapping translucent fills are already hard to read and three is unusable — that is what a stacked area chart solves.