Renamed members of ChartData

using "_" as prefix for private members to clarify what's hidden and
what's exposed.

* @data.ids
* @data.idsBySeries
* @data.varX
* @data.varsX
* @data.varsPivot
* @data.accessorFor(var)
* @data.provenanceAccessorFor(var)
* @data.relatedVarsFor(var)