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