customSymbol (customSymbol) {
        if (!arguments.length) {
            return this._symbol;
        }
        this._symbol = customSymbol;
        this._symbol.size((d, i) => this._elementSize(d, i));
        return this;
    }
Get or set the symbol generator. By default `ScatterPlot` will use
{@link https://github.com/d3/d3-shape/blob/master/README.md#symbol d3.symbol()}
to generate symbols. `ScatterPlot` will set the
{@link https://github.com/d3/d3-shape/blob/master/README.md#symbol_size symbol size accessor}
on the symbol generator.
@see {@link https://github.com/d3/d3-shape/blob/master/README.md#symbol d3.symbol}
@see {@link https://stackoverflow.com/questions/25332120/create-additional-d3-js-symbols Create additional D3.js symbols}
@param {String|Function} [customSymbol=d3.symbol()]
@returns {String|Function|ScatterPlot}