/* global d3 */
d3.selection.prototype.now = function(tagName, data, maker) {
    var key = function(v) { return v; };
    if (maker === undefined) {
        maker = data; data = [1];
    }
    var selection = this.selectAll(tagName)
        .data(data, key);
    selection.enter().append(tagName);
    selection.exit().remove();
    selection.each(function(d) {
        maker(d3.select(this), d);
    });
    return this;
};