Compute the Vega child-view name for one repeated cell. The rendered SVG
uses this name in the cell group class (<name>_group) and the mark
group class (<name>_marks), which is what makes repeat selectors
per-cell unique without any stamping.
Mirrors Vega-Lite's varName sanitisation: every non-word character in
a field name becomes _, and a field name starting with a digit gains a
leading _ (e.g. field 2020 → child___2020).
Compute the Vega child-view name for one repeated cell. The rendered SVG uses this name in the cell group class (
<name>_group) and the mark group class (<name>_marks), which is what makes repeat selectors per-cell unique without any stamping.Mirrors Vega-Lite's
varNamesanitisation: every non-word character in a field name becomes_, and a field name starting with a digit gains a leading_(e.g. field2020→child___2020).