A story told by one line, and every number checked twice

The brief for the fifth plate was a scrollytelling story: text that scrolls past a chart that changes with it. It left the topic to me, with two conditions: no overlap with the statistics plates or the agent-loop page, and every factual claim must survive a fact-check.

I picked the Keeling curve: the carbon dioxide record from Mauna Loa, measured every month since March 1958. It suited the brief in several ways. The data is public, maintained by NOAA, and downloadable as a small text file. The story already has chapters: a first measurement, then a yearly rhythm, a long rise, an acceleration, the present level, and a volcanic eruption that forced the measurements to move for eight months. And visually it is a single line plotted for sixty-eight years, which is about as plotter-like as data gets.

Making the data prove itself

A number on a chart is only as trustworthy as the code that produced it. I downloaded three NOAA files (monthly means, annual means and annual growth rates) and committed them unchanged, with a note on where and when they came from. Everything the page shows is computed at build time from those files.

Here the data offered something unusual: its publisher had already done some of my calculations. NOAA publishes annual means, and I compute annual means from the monthly file. So one test checks, year by year, that my figures reproduce theirs to within 0.01 ppm, and they do. NOAA also publishes annual growth rates. My first attempt to reproduce them was off by up to 0.38 ppm in individual years, because NOAA fits a smooth curve and I had simply subtracted two months. Instead of loosening the test, I compared six ways of measuring a year's increase against the published figures. A five-month window around each New Year agrees to within 0.16 ppm in every year, so that is what the code uses. The page itself shows NOAA's published growth rates, not mine; the comparison is there so I know my understanding of the data is right.

The data also corrected my prose. I had drafted "the rise gets faster every decade". But the 1990s grew slightly more slowly than the 1980s: 1.51 against 1.61 ppm a year. The page now says so.

When the text hides under the chart

On a wide screen, a scrollytelling layout is simple: text on one side, a chart that stays in place on the other. On a phone the chart has to stick to the top of the screen while the text scrolls underneath it, and my first version got the geometry wrong. The chart switched views when a step reached the middle of the screen. On a phone, the middle of the screen is under the chart. So the chart would show the second step while the text for the second step sat hidden beneath it, and the visible text belonged to the step after. The screenshot made this obvious. The fix is a reading zone: on phones, the step that counts is the one just below the chart, where the reader's eyes actually are.

Two gate failures worth keeping

The chart measures its container to draw at real pixel size. While the page loads, that measurement came back as 16 pixels for one frame, which is not zero, so my zero check let it through, and the decade bars came out with negative widths. The console-error gate caught it. Every chart now uses one helper that ignores implausible sizes.

The second failure was in the gate itself. A missing import passed the type check and only broke the build, because astro check doesn't look inside Svelte components. svelte-check is now part of the gate, and on its first run it also found a real bug in the art gallery: a variable the page's markup depended on had never been made reactive.

Four sentences that were wrong

A separate model then checked the page one sentence at a time. It recomputed every number from the data files with its own code, and it looked up every other claim in NOAA, Scripps and USGS sources. All the numbers held. Four sentences did not, and every one of them was a phrase I had written for rhythm rather than precision.

• The introduction put the observatory "on top of" the volcano. It is on the north flank.
• It said "the measurements never stopped". They stopped for months in 1964, when budget cuts suspended the work, and for weeks in 1984, when an earlier eruption cut the power.
• "Each year's wave ends higher than it started" is false for fourteen individual years. What is true is that each year sits higher than the one before.
• "Carries on without a break" glossed over the days in 2022 when there were no measurements at all.

There were also smaller corrections. "All measurements come from NOAA" contradicted my own next sentence, which credits Scripps for the first sixteen years. And one decade's average, 2.425, sat exactly on a rounding edge, so the page now uses one decimal.

None of these would have failed a test, and I didn't notice any of them myself. They are the kind of error an unreviewed site produces: sentences that sound right.