d3 stacked line graph

gistfile1.js

var width = 400;
var height = 300;
var len = 20;

var rand_data = function() {
  return d3.range(len).map(function() { return 1 + Math.random() });
}

var stack = d3.select("#chart").
  append("svg:svg").
  attr("width", width).
  attr("height", height);

var x = d3.scale.linear().domain([0, len]).range([0, width]);
var y = d3.scale.linear().domain([0, 6]).range([height, 0]);

var max_height = []
var area = d3.svg.area().
x(function(d, i) { return x(i) }).
  y0(function(d, i) {
    if (typeof max_height[i] !== "undefined") {
      return y(max_height[i])
    } else {
      return y(0)
    }
  }).
  y1(function(d, i) {
    if (typeof max_height[i] !== "undefined") {
      max_height[i] += d;
      return y(max_height[i]);
    } else {
      max_height[i] = d;
      return y(d)
    }
  }).
  interpolate("basis");


var line = d3.svg.line().
  x(function(d, i) { return x(i); }).
  y(function(d, i) { return y(d.y); });

var color = d3.interpolateRgb("#000", "#fff");
stack.selectAll("path").data([rand_data(), rand_data()]).enter().
  append("svg:path").
  attr("d", area).
  style("fill", function() { return color(Math.random()) });