symbol (type) {
        if (!arguments.length) {
            return this._symbol.type();
        }
        this._symbol.type(type);
        return this;
    }
Get or set the symbol type used for each point. By default the symbol is a circle (d3.symbolCircle).
Type can be a constant or an accessor.
@see {@link https://github.com/d3/d3-shape/blob/master/README.md#symbol_type symbol.type}
@example
// Circle type
chart.symbol(d3.symbolCircle);
// Square type
chart.symbol(d3.symbolSquare);
@param {Function} [type=d3.symbolCircle]
@returns {Function|ScatterPlot}