constructor () {
        this._htmlLegendDivCssClass = 'dc-html-legend';
        this._legendItemCssClassHorizontal = 'dc-legend-item-horizontal';
        this._legendItemCssClassVertical = 'dc-legend-item-vertical';
        this._parent = undefined;
        this._container = undefined;
        this._legendText = pluck('name');
        this._maxItems = undefined;
        this._horizontal = false;
        this._legendItemClass = undefined;
        this._highlightSelected = false;
        this._keyboardAccessible = false;
    }
htmlLegend is a attachable widget that can be added to other dc charts to render horizontal/vertical legend
labels.
@example
chart.legend(HtmlLegend().container(legendContainerElement).horizontal(false))
@returns {HtmlLegend}