Phase 6: Rendering cleanup — PlotContent_Category + PlotContent_XY

[timmolter]

Summary

Low-risk rendering cleanup targeting the two largest PlotContent_* classes. No logic changes — pixel output is identical before and after.

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Step 1: Rename PlotContent_Category_Bar → PlotContent_Category

The _Bar suffix was a historical artifact. The class renders all CategorySeries render styles (Bar, Stick, Line, Area, Scatter, SteppedBar). Updated the single instantiation site in Plot_Category.java.

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Step 2: Extract private methods from PlotContent_Category.doPaint()

The 490-line doPaint() is now a clean loop dispatcher. Extracted:

Method	What it does
computeBarDimensions(series, categoryCounter, gridStep, xLeftMargin, seriesCounter)	Calculates barWidth and xOffset for a given category/series position (was an anonymous braced scope)
processSteppedBarDataPoint(g, series, y, yOffset, zeroOffset, xOffset, gridStep, seriesCounter, isLastPoint, steppedPath, steppedReturnPath, previousY)	Accumulates one data point into the stepped-bar path; handles stacked sign-change mid-draw; returns updated previousY
finalizeSteppedBar(g, series, steppedPath, steppedReturnPath)	Draws the accumulated stepped-bar path after all data points for a series are processed

Key design decisions:

• Stacking accumulator arrays are passed as parameters, not fields
• steppedPath/steppedReturnPath pre-initialized as empty ArrayLists so they can be passed into extracted methods; isEmpty() replaces the null sentinel check
• drawLabels() was already properly extracted — left as-is

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Step 3: Extract paintErrorBar() from PlotContent_XY.doPaint()

The 47-line error bar block (top/bottom value + three Line2D segments) was self-contained. Extracted into private paintErrorBar(...).

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Verification

mvn test -pl xchart
Tests run: 73, Failures: 0, Errors: 0, Skipped: 0 — BUILD SUCCESS
mvn fmt:check -pl xchart — BUILD SUCCESS