* Rescale Y-axis on chart zoom via custom AxisProxy filterMode
Patch ECharts' AxisProxy.filterData to support a "boundaryFilter" mode
that keeps the nearest data point outside each zoom boundary while
filtering distant points. This lets ECharts natively rescale the Y-axis
to the visible data range without causing line gaps at the zoom edges.
* Add tests for ECharts AxisProxy patch internals
Verify that the ECharts internals our boundaryFilter patch relies on
still exist (filterData, getTargetSeriesModels on AxisProxy prototype),
and test the patch behavior: delegation for other filterModes, early
return for non-matching models, and correct boundary-preserving filtering.
* Update comment
