Summary

Closes #86 — adds the ability to define a tooltip (or label / axis-label / visualMap) formatter as a real JavaScript function built in Java, matching the JS API shown in the issue:

tooltip: {
  formatter(params) {
    return '<div>' + params.name + ': ' + params.value + ' (' + params.percent + '%)</div>';
  }
}

The string-template form ("{b}: {c}") already worked through the existing setFormatter(String) overloads. The function form was unreachable because Gson serialized any Object formatter as a quoted JSON string — the rendered HTML would contain a string, not a callable function.

How it works

A new JsFunction value class wraps a JS body. A low-level Gson TypeAdapter uses JsonWriter#jsonValue(String) to emit the body straight into the output stream, unquoted. When the option JSON is dropped into the rendered <script> tag (Engine uses Handlebars triple-braces, so no HTML-escaping), the function survives as a real JS callable.

The adapter is registered automatically in EChartsSerializer's constructor — no new public surface beyond JsFunction itself.

How to use

import org.icepear.echarts.serializer.JsFunction;

JsFunction formatter = new JsFunction(
    "function(p) { return p.name + ': ' + p.value + ' (' + p.percent + '%)'; }");

new Pie()
    .setTooltip(new Tooltip().setTrigger("item").setFormatter((Object) formatter))
    .addSeries(/* ... */);

The (Object) cast picks the existing setFormatter(Object) overload over setFormatter(String). Same pattern works on SeriesLabel, AxisLabel variants, PieLabel, and any other class with an Object-typed formatter.

For multi-line bodies just concatenate strings — newlines are preserved verbatim:

new JsFunction(
    "function(p) {"
  + "  return '<b>' + p.name + '</b>: ' + p.value;"
  + "}");

⚠️ Important: JsFunction-bearing output is meant for embedding in HTML/JS. The result is valid JavaScript but not strictly valid JSON — don't feed it into a JSON parser. Body validation is the browser's job; garbage syntax surfaces as a JS error in the console.

Tests added (18 new, all passing)

serializer/JsFunctionTest covers:

• Value class — equals/hashCode, null-body rejection, static factory parity
• Unquoted-output invariant — top-level, nested in object, nested in array
• String formatters remain quoted (no regression)
• Multi-line bodies preserved exactly
• HTML <tag> characters preserved (HTML escaping stays disabled, critical for tooltip markup)
• End-to-end through Tooltip on Pie and Bar
• Sanity check that the embedded result starts with f (function), not "
• Read direction (best-effort symmetric round-trip)
• Body-validation is explicitly out-of-scope and won't throw

Visual demo

src/test/java/org/icepear/echarts/demo/JsFunctionTooltipDemo.java builds a richer multi-element tooltip (using params.marker, params.name, params.value, params.percent) on a Pie chart and writes a self-contained dark-themed HTML to /tmp/js-function-tooltip-demo.html.

mvn test -Dtest=JsFunctionTooltipDemo
open /tmp/js-function-tooltip-demo.html
# Hover over a slice — the rich HTML tooltip is rendered by the JS function built in Java.

Test plan

• mvn test -Dtest=JsFunctionTest — 18/18 pass
• mvn test -Dtest=JsFunctionTooltipDemo — writes demo file
• open /tmp/js-function-tooltip-demo.html — tooltip renders with rich HTML on hover; verified the rendered HTML contains "formatter":function (params) { (unquoted)
• mvn test (full suite) — 95 tests, only the 3 pre-existing polar/float-precision failures remain on master