Add Chart.setSize so a chart can be resized before export

[timmolter]

Summary

Adds a public, fluent setSize(int width, int height) to Chart. Until now a chart's dimensions could only be set via the builder, so a chart constructed by code outside the caller's control (the case in #807) could not be exported at a chosen size. The existing setWidth/setHeight are protected and stay that way.

chart.setSize(1920, 1080);
ChartEncoder.saveChart(chart, "./MyChart", "png");

Every exporter (BitmapEncoder, VectorGraphicsEncoder, PdfboxGraphicsEncoder) already reads getWidth()/getHeight() from the chart, so no encoder changes are needed and all formats pick up the new size. Non-positive dimensions throw IllegalArgumentException.

This is an alternative to #1034, which adds width/height overloads to ChartEncoder. A single setter on the chart covers the same use case with less API surface, makes the size change explicit rather than a side effect of exporting, and also helps anyone who wants to resize a chart before handing it to a Swing panel.

Changes

• Chart.setSize(int, int) returning this
• ChartSizeTest: chaining, validation, PNG dimensions, SVG viewBox, PDF page size, pie and category charts
• TestForIssue807 demo
• README note under Exporting Charts

Test plan

• mvn -pl xchart test: 180 tests, 0 failures
• xchart-demo compiles

Fixes #807

🤖 Generated with Claude Code