nowCastAqiSeries

The historical NowCast AQI for every reading in readings — element i is the AQI as of readings[i]'s own timestamp, derived only from readings at or before it, or null where EPA's minimum-data rule (see computeNowCastPm25) isn't met at that point. Used to chart/tabulate AQI over time (issue #6381) rather than only the single live value.

readings must be sorted ascending by epoch-second timestamp; a sliding window keeps this linear in the number of readings per 12-hour window rather than quadratic over the whole time frame.