SVG grouping component with z-index convention for chart layering.
Inspired by Recharts' z-index layering — wraps chart elements in <g> groups
with documentary z-values. SVG paints in DOM order, so the parent must sort
layers by z-value before rendering.
Standard z-values (Recharts convention)
- grid: -100
- area: 100
- bar: 300
- line: 400
- axis: 500
- scatter: 600
- label: 2000
Examples
<.chart_layer z={-100}>
<%!-- grid lines --%>
</.chart_layer>
<.chart_layer z={400}>
<%!-- line series --%>
</.chart_layer>