tickFormat (tickFormat) {
        if (!arguments.length) {
            return this._tickFormat;
        }
        this._tickFormat = tickFormat;
        return this;
    }
Get or set the numerical format of the boxplot median, whiskers and quartile labels. Defaults
to integer formatting.
@example
// format ticks to 2 decimal places
chart.tickFormat(d3.format('.2f'));
@param {Function} [tickFormat]
@returns {Number|Function|BoxPlot}