The archive
The same series that fails to explain prices establishes something about itself: the capitals are warming, and heating demand is falling with them.
Fitted across 19 complete years, the mean temperature of these capitals rises 0.69 °C per decade, and annual heating degree days fall by 16.4 a year. Fitted per country instead of on the pooled average, 40 of 41 capitals are warming.
The R² is 0.54 on that pooled average, which is a real trend with a lot of weather noise on top of it — about what annual observations over this span can support, and no more.
Bars rather than a line is not a style choice. The years here are whichever ones have been fetched, a line chart interpolates across any that have not, and an invented segment between two real observations is indistinguishable from data. Each missing year still gets its slot on the axis, empty, so 2007 does not sit beside 2012 as if they were consecutive.
The current year is not comparable
The archive stops a few days short of today, so the current year is always partial and an annual degree-day total over a partial year is not comparable with a whole one.
As of 2026-10-19 the current year holds 292 days and 1,331 heating degree days, which is 70.4% of last complete year's total. Charted beside the complete years it would read as a collapse in heating demand, and every chart in the weather section filters it out on a flag rather than trimming a year off by hand.