feat(chart): add MONOTONE line interpolation via LineInterpolation enum#206
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Replace the boolean smooth flag on ChartSpec.Line with a LineInterpolation enum (LINEAR / SMOOTH / MONOTONE) — one explicit knob for line shape. The new MONOTONE mode emits a Fritsch-Carlson monotone cubic: as smooth as the Catmull-Rom SMOOTH curve but constrained to never overshoot, so the line stays within the value range of the points it spans. All modes render through the same native PDF curve operators with zero tessellation, so geometry stays deterministic and the layout hot path is unchanged.
…NE comparison The hero preview now shows the two volatile-price cards stacked: SMOOTH crests above the data's peak (overshoot) while MONOTONE stays within the value range — the accuracy difference the new interpolation mode adds.
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Why
Smooth (Catmull-Rom) line charts look good but can overshoot local extremes — on sharp value swings the curve briefly leaves the range of the data it connects, so it claims values the data never had. There was no smooth-yet-accurate option: straight segments are exact but angular, the smooth curve is pretty but inaccurate.
What changed
LineInterpolationenum (LINEAR/SMOOTH/MONOTONE) replaces the booleansmoothflag onChartSpec.Line— a single explicit knob for line shape.MONOTONEemits a Fritsch-Carlson monotone cubic inLineChartLayout: as smooth asSMOOTH, but each span is constrained to stay within the value range of its two endpoints, so the curve never overshoots and preserves the data's rises and falls.PathNodeper run, zero tessellation — so geometry stays deterministic and the layout hot path is unchanged.smooth(true)→interpolation(LineInterpolation.SMOOTH)). The chart API is unreleased (@since 1.8.0), so no deprecation bridge was needed.ChartShowcaseExamplegains a SMOOTH-vs-MONOTONE comparison on a volatile series; the committedassets/readme/examples/chart-showcase.pdfpreview is refreshed.Verification
./mvnw test -pl .→ 1365 tests, 0 failures.ChartLayoutResolverTest.monotoneCurveStaysWithinTheDataRangeWhileSmoothOvershoots— proves the monotone curve's bounding box stays inside the LINEAR data range (ground truth) while SMOOTH bulges past the plateau.monotoneLineEmitsOneNativeBezierRun+ layout snapshotcharts/line_monotone.