selectAll (sel) {
        return this._root ? this._root.selectAll(sel) : null;
    }
Execute in scope d3 selectAll using the given selector and return d3 selection result.

This function is **not chainable** since it does not return a chart instance; however the d3
selection result can be chained to d3 function calls.
@see {@link https://github.com/d3/d3-selection/blob/master/README.md#selectAll d3.selectAll}
@example
// Has the same effect as d3.select('#chart-id').selectAll(selector)
chart.selectAll(selector)
@param {String} sel CSS selector string
@returns {d3.selection}