nonemptyOpacity (nonemptyOpacity) {
        if (!arguments.length) {
            return this._emptyOpacity;
        }
        this._nonemptyOpacity = nonemptyOpacity;
        return this;
    }
Set or get opacity for symbols when the group is not empty.
@param {Number} [nonemptyOpacity=1]
@return {Number}
@return {ScatterPlot}