withTrend method
Replaces TrendAnnotation.trendType and TrendAnnotation.windowSize together.
The parameters are assert-coupled, so they only move as a pair.
No clear verb: this class's copyWith cannot unset them; construct a new instance to reset.
Implementation
TrendAnnotation withTrend(TrendType trendType, int windowSize) =>
copyWith(trendType: trendType, windowSize: windowSize);