x = function(d, _) {
        if(arguments.length < 2) return d.x;
        d.x = _;
        return accessor;
      }
Supports getter and setter. Watch out if used in d3 and the second parameter is an index!!
This approach needs further thought.
@param d Underlying data object to get or set the value
@param _ If passed turns into a setter. This is the value to set
@returns {*}