Hover any dot for exact value Filled area = cumulative volume beneath the trend
An Area Graph is a line chart with the region between the line and the baseline filled in. The fill directs the viewer's attention to cumulative volume beneath the curve rather than just the trajectory of individual data points. Where a plain line chart asks "is this going up or down?", an area graph asks "how much has accumulated?" — making it ideal for showing total magnitude, running totals, and the proportional weight of a trend over time.
This example shows fictional monthly research hours across a twelve-month period. The upward trend from 28 hours in January to a peak of 90 in September illustrates a common project pattern: slow ramp-up, peak productivity mid-cycle, modest year-end taper. The filled area makes the cumulative investment visible — the viewer sees that the entire second half of the year carries the bulk of total effort, something a bare line cannot communicate as directly.
An area graph encodes a single continuous time series using two visual channels simultaneously: the line's position (height at any x) encodes the point value, and the filled area between the line and the zero baseline encodes cumulative mass. The perceptual mechanism exploited is area estimation — the human visual system integrates the filled region as a quantity, making the total volume beneath the curve legible without arithmetic. The gradient fill amplifies this: the colour is strongest at the line and fades toward the baseline, directing the eye upward to the peak and reinforcing the reading that high-valued periods are "heavier" than low-valued ones.
The dataset is a single continuous time series — monthly research hours — and the message is about the cumulative weight of effort across the year, not just whether any individual month is high or low. The area encoding makes that weight physically present on the page. A plain line chart would show the trajectory equally well but would lose the sense of total accumulation. A bar chart would show individual months discretely but fragment the continuity of the trend. The area graph's combination of line continuity and filled mass is the correct encoding for this data and message.
A stacked area chart is the closest structural relative — it works when multiple series need to be shown simultaneously while preserving cumulative volume. But multiple overlapping unfilled area charts are the classic failure mode: when two series cross, the lower series disappears behind the upper one's fill. The area graph is therefore strictly appropriate for single-series data, or for cases where series are stacked rather than overlapped. A non-zero y-axis baseline is the other critical failure mode: if the area doesn't start at zero, the filled region misleads — its visual mass implies a total that doesn't correspond to the actual quantity.
FT Visual Vocabulary category: Change Over Time — "Show change in a single variable across an ordered sequence, here emphasising total magnitude through area fill." Tufte principle: area fill adds a second data-ink channel without adding chart junk — every pixel of the filled region encodes real quantity. The one design decision worth knowing: the y-axis baseline is fixed at zero. Moving it to any positive value would make the filled area proportionally larger than the actual data range, creating a visual lie factor greater than one.