title (titleFunction) {
        if (!arguments.length) {
            return this._title;
        }
        this._title = titleFunction;
        return this;
    }
Set or get the title function. The chart class will use this function to render the SVGElement title
(usually interpreted by browser as tooltips) for each child element in the chart, e.g. a slice
in a pie chart or a bubble in a bubble chart. Almost every chart supports the title function;
however in grid coordinate charts you need to turn off the brush in order to see titles, because
otherwise the brush layer will block tooltip triggering.
@example
// default title function shows "key: value"
chart.title(function(d) { return d.key + ': ' + d.value; });
// title function has access to the standard d3 data binding and can get quite complicated
chart.title(function(p) {
   return p.key.getFullYear()
       + '\n'
       + 'Index Gain: ' + numberFormat(p.value.absGain) + '\n'
       + 'Index Gain in Percentage: ' + numberFormat(p.value.percentageGain) + '%\n'
       + 'Fluctuation / Index Ratio: ' + numberFormat(p.value.fluctuationPercentage) + '%';
});
@param {Function} [titleFunction]
@returns {Function|BaseMixin}