select (sel) {
        return this._root.select(sel);
    }
Execute d3 single selection in the chart's scope using the given selector and return the d3
selection.

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#select d3.select}
@example
// Has the same effect as d3.select('#chart-id').select(selector)
chart.select(selector)
@param {String} sel CSS selector string
@returns {d3.selection}