valueAccessor (valueAccessor) {
        if (!arguments.length) {
            return this._valueAccessor;
        }
        this._valueAccessor = valueAccessor;
        return this;
    }
Set or get the value accessor function. The value accessor function is used to retrieve the
value from the crossfilter group. Group values are used differently in different charts, for
example values correspond to slice sizes in a pie chart and y axis positions in a grid
coordinate chart.
@example
// default value accessor
chart.valueAccessor(function(d) { return d.value; });
// custom value accessor for a multi-value crossfilter reduction
chart.valueAccessor(function(p) { return p.value.percentageGain; });
@param {Function} [valueAccessor]
@returns {Function|BaseMixin}