existenceAccessor (accessor) {
        if (!arguments.length) {
            return this._existenceAccessor;
        }
        this._existenceAccessor = accessor;
        return this;
    }
Get or set the existence accessor.  If a point exists, it is drawn with
{@link ScatterPlot#symbolSize symbolSize} radius and
opacity 1; if it does not exist, it is drawn with
{@link ScatterPlot#emptySize emptySize} radius and opacity 0. By default,
the existence accessor checks if the reduced value is truthy.
@see {@link ScatterPlot#symbolSize symbolSize}
@see {@link ScatterPlot#emptySize emptySize}
@example
// default accessor
chart.existenceAccessor(function (d) { return d.value; });
@param {Function} [accessor]
@returns {Function|ScatterPlot}