Unified ChartEncoder + optional export deps (#772)

[timmolter]

Summary

Adds a single, format-agnostic export facade and makes the heavy export dependencies genuinely optional. Motivated by #772 (AVIF export): rather than hard-code each format, export becomes pluggable through javax.imageio.ImageIO.

ChartEncoder.saveChart(chart, "./MyChart", "png");   // raster via ImageIO
ChartEncoder.saveChart(chart, "./MyChart", "tiff");  // raster, built into the JDK
ChartEncoder.saveChart(chart, "./MyChart", "webp");  // raster, via any ImageIO WebP plugin
ChartEncoder.saveChart(chart, "./MyChart", "svg");   // vector
ChartEncoder.saveChart(chart, "./MyChart", "pdf");   // vector
byte[] bytes = ChartEncoder.getBytes(chart, "png");

What changed

• ChartEncoder — new facade. svg/eps/pdf route to the existing vector encoders; every other format name is delegated to ImageIO, so any registered ImageWriter plugin (WebP, AVIF, …) works with no XChart code change. Includes getBytes(...) and getSupportedRasterFormats().
• Optional deps fixed — VectorGraphics2D, pdfbox-graphics2d and animated-gif-lib are now marked <optional>true</optional> in the module pom. They were declared optional only in the parent <dependencyManagement>, where Maven does not inherit the flag, so they were being pulled transitively into every consumer. The demo module now declares them explicitly.
• Friendly errors — Utils.requireOnClasspath(...) makes a missing optional encoder throw an IllegalStateException naming the exact artifact to add, instead of a raw NoClassDefFoundError.
• Deprecations — the per-format saveBitmap / saveVectorGraphic / savePdfboxGraphics methods are deprecated in favor of ChartEncoder. saveBitmapWithDPI and saveJPGWithQuality remain (no facade equivalent).
• Samples — Example1 uses ChartEncoder and emits a TIFF sample; new ExampleWebP (via com.github.usefulness:webp-imageio, demo-only) and ExampleAvif (via the libavif avifenc CLI) generate WebP/AVIF samples. Size story that motivated Export in AVIF #772: PNG 15.7 KB → WebP 7.1 KB → AVIF 5.4 KB.
• Tests / docs — ChartEncoderTest covers every available raster format, the vector formats, byte output, extension handling, and the unsupported-format error. README gains an "Exporting Charts" section.

Notes / caveats

• ⚠️ Behavior change for consumers: because the export libs are no longer transitive, projects that use PDF/SVG/EPS/GIF export must now add the matching dependency themselves. Worth a release-note. Fits the 4.x line.
• AVIF is not yet an in-library format. There is no bundleable Maven-Central ImageIO AVIF writer (vavi isn't on Central; the nemanjastokuca plugin is reader-only; JDeli is commercial). ExampleAvif therefore uses the avifenc CLI as a portable fallback. If/when a viable AVIF ImageWriter exists, ChartEncoder.saveChart(chart, "avif") will work with no code change.
• The demo's WebP plugin is com.github.usefulness:webp-imageio (maintained fork with macOS arm64 natives); the older gotson fork ships no Apple-Silicon binary.

Full build green: 122 tests pass, demo + site build.

Closes #772

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR introduces a unified export façade (ChartEncoder) that routes vector formats to existing encoders and delegates all raster formats to javax.imageio.ImageIO, enabling new raster formats (e.g., WebP) via ImageIO plugins without XChart code changes. It also corrects Maven dependency optionality so heavy export dependencies are no longer pulled transitively by default, and adds tests/demos/docs around the new API.

Changes:

• Added ChartEncoder with saveChart(...), getBytes(...), and getSupportedRasterFormats(); raster export is ImageIO/SPI-driven and vector export is routed to existing encoders.
• Marked vector/PDF/animated-GIF export dependencies as truly optional in the xchart module POM; demo declares them explicitly.
• Added Utils.requireOnClasspath(...), new demos (WebP + AVIF fallback), new unit tests, and README updates; deprecated older per-format entry points.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
xchart/src/test/java/org/knowm/xchart/ChartEncoderTest.java	New tests covering raster/vectors, bytes output, extension handling, and unsupported-format errors.
xchart/src/main/java/org/knowm/xchart/VectorGraphicsEncoder.java	Deprecates old entry points and adds classpath-guard logic for optional vector deps.
xchart/src/main/java/org/knowm/xchart/PdfboxGraphicsEncoder.java	Deprecates old PDF encoder entry points and adds a classpath check for PDF export dependency.
xchart/src/main/java/org/knowm/xchart/internal/Utils.java	Adds requireOnClasspath(...) helper for friendly missing-dependency errors.
xchart/src/main/java/org/knowm/xchart/GifEncoder.java	Adds a classpath check for the animated GIF dependency before use.
xchart/src/main/java/org/knowm/xchart/ChartEncoder.java	New unified export façade (vector routing + ImageIO raster delegation).
xchart/src/main/java/org/knowm/xchart/BitmapEncoder.java	Deprecates legacy bitmap save/get-bytes entry points in favor of ChartEncoder.
xchart/pom.xml	Marks VectorGraphics2D / pdfbox-graphics2d / animated-gif-lib as <optional>true</optional> at the module level.
xchart-demo/src/main/java/org/knowm/xchart/standalone/ExampleWebP.java	New demo showing WebP export via ImageIO plugin + ChartEncoder.
xchart-demo/src/main/java/org/knowm/xchart/standalone/ExampleAvif.java	New demo showing AVIF generation via PNG + avifenc CLI fallback.
xchart-demo/src/main/java/org/knowm/xchart/standalone/Example1.java	Updates sample to use ChartEncoder for unified export and adds TIFF output.
xchart-demo/pom.xml	Adds explicit demo deps for optional export libs + demo-only WebP ImageIO plugin.
README.md	Updates examples and adds “Exporting Charts” documentation with optional dependency table.
.gitignore	Ignores additional generated export artifacts (tif/tiff/webp/avif).