Fix #125: TeX/LaTeX formula rendering in chart titles, axis titles, tick labels, and legend entries

[timmolter]

Summary

Adds LaTeX math formula rendering to all chart text elements via JLatexMath.

Any string wrapped in $...$ is rendered as a display-mode LaTeX formula. Strings without the $ wrapper continue to use the existing TextLayout path unchanged — no behaviour change for existing charts.

XYChart chart = new XYChartBuilder()
    .title("$\\hat{y} = \\beta_0 + \\beta_1 x$")
    .xAxisTitle("$x$ (time, $\\mu s$)")
    .yAxisTitle("$y$ (amplitude)")
    .build();

chart.addSeries("$\\alpha$-channel", xData, yData);

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Licensing — why this is safe now

JLatexMath is GPL v2. The original blocker (noted in this issue in 2016) was that the GPL licence is incompatible with XChart's Apache 2.0 without the Classpath Exception (linking exception).

That exception was added to JLatexMath in commit f173337 on 2016-01-19 with the message:

"Change licence by adding linking exception. Discussed here: https://forge.scilab.org/index.php/p/jlatexmath/issues/1566/"

That scilab issue #1566 is exactly the thread @timmolter referenced in his 2016 comment when he wrote:

"They need to add the part about 'GPL with classpath exception' to their LICENSE… Until that exists, the license is incompatible with XChart's."

The condition was met ~ten years ago. JLatexMath 1.0.7 has been on Maven Central since then with the Classpath Exception present.

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Optional dependency

JLatexMath is declared <optional>true</optional> in xchart/pom.xml — exactly like VectorGraphics2D. Users who don't need TeX rendering do not need to add anything to their build.

If JLatexMath is not on the classpath, TexRenderer.isAvailable() returns false and $...$ strings fall back to plain text (no exceptions).

To opt in, add to your own pom.xml:

<dependency>
    <groupId>org.scilab.forge</groupId>
    <artifactId>jlatexmath</artifactId>
    <version>1.0.7</version>
</dependency>

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Implementation details

File	Change
pom.xml	Add jlatexmath 1.0.7 to <dependencyManagement>
xchart/pom.xml	Declare jlatexmath as optional dep
TexRenderer.java (new)	Central utility: isTeX(), getBounds(), render(), renderRotated() with runtime guard
ChartTitle.java	Delegate sizing + paint to TexRenderer
AxisTitle.java	X and Y axis titles; Y-axis rotation via BufferedImage intermediate
AxisTickLabels.java	Per-tick sizing + paint for both axes
Legend_.java	getSeriesTextBounds() + paintSeriesText()
TestForIssue125.java (new)	Demo: XY chart with TeX in all four text areas + category chart with TeX tick labels
TexRendererTest.java (new)	10 unit tests: detection, bounds, fallback

Coordinate system note

TeXIcon uses top-left origin while TextLayout uses a baseline origin. TexRenderer compensates so TeX and plain-text elements align consistently.

Rotated Y-axis title

JLatexMath's paintIcon doesn't compose cleanly with pre-applied AffineTransform rotations. The Y-axis title is therefore rendered to an off-screen BufferedImage, then drawn with a −90° rotation — correct orientation with no rendering artefacts.

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Tests

Tests run: 60, Failures: 0, Errors: 0, Skipped: 0

mvn fmt:check passes with no violations.

Closes #125

[timmolter]

Closing to rethink integration approach before re-opening. TexRenderer utility and demo are working; need to reconsider the minimal-footprint integration into core rendering classes.