Summary

• Adds the lines series — draws lines/curves between coordinate pairs. The standard pick for flight paths, geo flows, and migration visualizations. Pairs naturally with map (PR Add Map chart support (closes #100) #101) and effectScatter (PR Add EffectScatter chart support #102) for the canonical "animated flows on a map" pattern.
• Lines extends the base Chart (not CartesianCoordChart), so empty xAxis / yAxis arrays aren't auto-emitted — important when the user is using coordinateSystem: "geo". For cartesian2d usage, axes can be wired via Option directly.

Public API added

• Lines
• charts.lines.LinesSeries, LinesDataItem, LinesEmphasis, LinesEffect
• origin.chart.lines.LinesSeriesOption, LinesStateOption, LinesEmphasisOption, LinesDataItemOption, LinesEffectOption

How to use

Cartesian (no map needed)

Lines chart = new Lines()
    .setTitle("Flight paths")
    .addSeries(new LinesSeries()
        .setCoordinateSystem("cartesian2d")
        .setPolyline(false)
        .setEffect(new LinesEffect().setShow(true).setSymbol("arrow").setTrailLength(0.7))
        .setData(new LinesDataItem[] {
            new LinesDataItem().setCoords(new Number[][] { {0, 0}, {50, 80} }),
            new LinesDataItem().setCoords(new Number[][] { {50, 80}, {120, 30} })
        }));

Geo (paired with the map series from PR #101)

new LinesSeries()
    .setCoordinateSystem("geo")
    .setEffect(new LinesEffect().setShow(true).setSymbol("arrow").setTrailLength(0.6))
    .setLineStyle(new LineStyle().setColor("#a6c8ff").setWidth(1).setOpacity(0.6))
    .setData(/* lng/lat pairs */);

For a runnable visual demo with 5 "flight paths" on a dark canvas, see src/test/java/org/icepear/echarts/demo/LinesDemo.java — run mvn test -Dtest=LinesDemo then open /tmp/lines-demo.html.

Tests added (25 new, all passing)

• simple/lines/BasicLinesTest — snapshot
• charts/lines/LinesSeriesTest — 21 unit tests covering every setter overload, all three coordinate systems, effect nesting, lineStyle nesting, emphasis flags, the no-empty-axis-arrays invariant, null-omission, and round-trip serialization
• render/simple/RenderLinesByChartTest — Engine smoke test
• demo/LinesDemo — opt-in HTML writer for visual inspection

Test plan

• mvn test -Dtest=BasicLinesTest — 1/1
• mvn test -Dtest=LinesSeriesTest — 21/21
• mvn test -Dtest=RenderLinesByChartTest — 2/2
• mvn test -Dtest=LinesDemo then open /tmp/lines-demo.html — animated arrows render correctly
• mvn test (full suite) — 101 tests, only the 3 pre-existing polar/float-precision failures remain