title (stackName, titleAccessor) {
        if (!stackName) {
            return super.title();
        }

        if (typeof stackName === 'function') {
            return super.title(stackName);
        }
        if (stackName === this._groupName && typeof titleAccessor === 'function') {
            return super.title(titleAccessor);
        }

        if (typeof titleAccessor !== 'function') {
            return this._titles[stackName] || super.title();
        }

        this._titles[stackName] = titleAccessor;

        return this;
    }
Set or get the title function. Chart class will use this function to render svg title (usually interpreted by
browser as tooltips) for each child element in the chart, i.e. a slice in a pie chart or a bubble in a bubble chart.
Almost every chart supports title function however in grid coordinate chart you need to turn off brush in order to
use title otherwise the brush layer will block tooltip trigger.

If the first argument is a stack name, the title function will get or set the title for that stack. If stackName
is not provided, the first stack is implied.
@example
// set a title function on 'first stack'
chart.title('first stack', function(d) { return d.key + ': ' + d.value; });
// get a title function from 'second stack'
var secondTitleFunction = chart.title('second stack');
@param {String} [stackName]
@param {Function} [titleAccessor]
@returns {String|StackMixin}