What it is
An area chart draws a line and fills the region between it and the axis. The fill makes the magnitude of a single series feel tangible — total visitors, rainfall, energy produced — while the top edge still shows the trend.
With two or three series the areas overlap, so Chart Buffet keeps the fills translucent. For parts of a total, use a stacked area chart instead.
✓ Use it when
- You have one series over time and want its size, not just its direction, to register.
- You are comparing two or three series whose overlap you want to see.
✗ Avoid it when
- You have many series — overlapping fills turn to mud. Use lines.
- The values can be negative, or the axis cannot start at zero: the filled area then misrepresents the quantity.
Tips for a better chart
- Always start the value axis at zero; the filled area is meant to be proportional to the value.
- Keep the fill light and the top line strong.
- Put the larger series behind the smaller one if they overlap.
The maths
For a series measured over time, the filled area approximates the integral ∫ y dt: with straight segments it is the trapezoid sum Σ (xi+1 − xi) · (yi + yi+1)/2. For a rate (visitors per day) that area is the cumulative total, which is why the fill feels like “volume”.
A little history
Playfair shaded the space between his import and export lines in 1786 to show the balance of trade, one of the first uses of a filled area to carry meaning (Playfair 1786). Area charts became common in the twentieth century’s financial and scientific graphics, and remain popular in dashboards where a single trend needs presence.
Questions
Area chart or line chart?
A line when the trend is the point or there are several series; an area when one series’ size over time should register.
References
- Playfair, W. (1786). The Commercial and Political Atlas. London: Debrett, Robinson and Sewell.
- Tufte, E. R. (1983). The Visual Display of Quantitative Information. Cheshire, CT: Graphics Press.
- Friendly, M. & Denis, D. J. (2001). Milestones in the history of thematic cartography, statistical graphics, and data visualization. Web document.