d3.selectAll(selector)
Selects all elements that match the specified selector string. The elements will be selected in document order (top-to-bottom). If no elements in the document match the selector, or if the selector is null or undefined, returns an empty selection. For example, to select all paragraphs:
var paragraph = d3.selectAll("p");
If the selector is not a string, instead selects the specified array of nodes; this is useful if you already have a reference to nodes, such as this.chi