gistfile1.js

function chart() {
  this.init();
}
_.assign(chart.prototype, {
  xAxis: function(opt) {
    if (!val) { return this.state.xAxis; }
    this.state.xAxis = _.assign(defaultXAxisOptions, opt);
    return this;
  }
});

function lineChart() {
  chart.apply(this);
}

_.assign(lineChart.prototype, {
  init: function() {
    return this
  }
});