_legendItemHeight () {
        return this._gap + this._itemHeight;
    }
If set, individual legend items will be focusable from keyboard and on pressing Enter or Space
will behave as if clicked on.

If `svgDescription` on the parent chart has not been explicitly set, will also set the default 
SVG description text to the class constructor name, like BarChart or HeatMap, and make the entire
SVG focusable.
@param {Boolean} [keyboardAccessible=false]
@returns {Boolean|Legend}