Model: Series.trendlines (Vec<Trendline>, serde default) with kind
(linear, exponential, logarithmic, polynomial 2..6, power, moving
average 2..255), name, forward/backward forecast, set intercept,
equation / R² display and line colour, width and dash.
XLSX: read and write <c:trendline> in CT_Trendline child order, placed
after dLbls and before the data references in line, bar, area, scatter,
bubble and stock series; unknown types are skipped.
Chart: least-squares fits (transformed for exp/log/power, skipping
non-positive values; polynomials via normal equations on centred,
scaled x with partial pivoting), moving averages, forecasts that widen
the category or x axis, clipping to the plot, equation and R² labels and
legend entries ("Linear (Sales)"). Degenerate data draws nothing.
Engine: chart.addElement element="trendline" adds (type, series, order,
period, forward, backward, displayEquation, displayRSquared) or removes
trendlines; document.inspect lists each chart's trendlines.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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