Size & scale

The stack with one baseline

One band starts at zero. The rest float on whatever is underneath them.

a.k.a. stacked area chart · stacked bars · the 100% stack · share-of-total chart · the floating band · composition chart

Stacking pours the series into one silhouette, and only the one at the bottom keeps a baseline. Every band above it is the gap between two moving edges, so reading one means subtracting two positions the chart never prints — and where the stack climbs steeply the eye measures the shortest way across the band rather than the vertical way, which shaves it thinner still. Then there is the variant that runs the axis to 100%, and it is the one that does real damage: the total is divided out, so nothing on the page is an amount any more. A series that tripled can only be drawn as a narrowing band if the series beneath it grew faster, and narrowing is what the reader remembers.

How to spot it

Count the baselines. There is one, at the bottom. Every other band’s zero is wherever the band below it happened to end.
Pick a band that isn’t the bottom one and try to read its value at a given year. If it takes a ruler and a subtraction, the chart is not showing you that number.
Read the axis. If it ends at 100%, no band on it is a quantity, and no band can rise unless another falls.
A band that “shrinks” while the total grows: check whether it shrank, or merely grew more slowly than the thing underneath it.
Ask who got the bottom slot. That series is the only one drawn from zero, and somebody chose it.
On a steep climb, cover the bands below with your hand. The one you were reading is usually fatter than it looked.

The fix

Unstack it. One panel per series, each with its own zero baseline and its own axis in the unit people actually care about — then every series is a length again, and a component that grew is drawn growing. If the total is part of the story, plot the total as its own line alongside the parts rather than asking a silhouette to carry both jobs at once. Keep the stack only where the question genuinely is “how is the whole divided”, put the series you most want read at the bottom, and print the numbers, because a band the reader cannot measure is a band the reader will guess at. And when a share moves, say in the caption which way the level moved, since those are two different findings and only one of them is on the axis.

In the gallery

Dark navy briefing slide, masthead “Efficiency Desk · U.S. data centers · Where the electricity goes”, headlined “Cooling overhead is being engineered out”. A subhead reads: Berkeley Lab’s national model of the entire U.S. data center stock, at the three years the report publishes; every share below is the figure in the report, on an axis running the full 0% to 100%. Under the label “Share of total U.S. data center electricity · by equipment category”, a two-band stacked area runs across a zero-to-one-hundred-percent axis marked 0%, 25%, 50%, 75% and 100%, with three anchor points at 2018, 2024 and 2030. The lower blue band is labelled “IT equipment — servers, storage, network”; the gold band above it is labelled “Facility infrastructure — cooling & power distribution”, and carries the figures 36% at 2018, 31% at 2024 and 26% at 2030, a white boundary line rising gently from left to right as the gold band thins. The 2018 and 2024 columns are marked historical and the 2030 column Reference Case. A strip beneath the axis reads “Average PUE, all U.S. data centers, 1.55 → 1.45 → 1.36 — overhead per unit of IT load, falling every year”. Down the right, a card carries 26% in large white type over the words “of U.S. data center electricity goes to cooling and power distribution in 2030 — against 36% in 2018”. A second card, “The overhead band, year by year”, lists 2018 historical at 36%, 2024 historical at 31% and 2030 Reference Case at 26%. A gold-bordered note reads: “Ten points of overhead gone in twelve years. The cooling plant is being designed out of the data center,” with a small grey disclosure beneath it, “Shares of total data center electricity. Totals are published separately.” Three ticked boxes run along the bottom: “Every share published — Berkeley Lab’s 2025 Update, Section 3.6, nothing interpolated between the years shown”, “One definition throughout — infrastructure is cooling plus power-distribution losses, the same quantity PUE measures”, and “Full axis — 0% to 100%, no truncation, no log scale, no second axis”. A footnote credits the Berkeley Lab report LBNL-2001758 of June 2026 for the shares and the PUE figures, notes that the 2030 share is PUE minus one over PUE at the Reference Case PUE of 1.36, and calls the slide an Efficiency Desk demonstration drawn by us.

The cooling band that shrank while the cooling grew

The stack with one baseline