Generates a CSS selector for a series' rendered group element — the
<g class="highcharts-series highcharts-series-N"> wrapping all of the
series' marks inside .highcharts-series-group.
Used as the per-panel MaidrSubplot.selector: MAIDR's subplot-layout pass
measures this element's bounding box to derive the panels' visual order and
the vertical arrow-key direction (Highcharts SVG has no g[id^="axes_"]
groups, so without a measurable per-panel element the core falls back to
data order and Up/Down are inverted for multi-row grids). The first layer's
selectors are not a reliable substitute — box, candlestick, and heatmap
layers carry structured selector objects the layout pass cannot query.
Generates a CSS selector for a series' rendered group element — the
<g class="highcharts-series highcharts-series-N">wrapping all of the series' marks inside.highcharts-series-group.Used as the per-panel
MaidrSubplot.selector: MAIDR's subplot-layout pass measures this element's bounding box to derive the panels' visual order and the vertical arrow-key direction (Highcharts SVG has nog[id^="axes_"]groups, so without a measurable per-panel element the core falls back to data order and Up/Down are inverted for multi-row grids). The first layer's selectors are not a reliable substitute — box, candlestick, and heatmap layers carry structured selector objects the layout pass cannot query.