old_contents: var margin = {top: 20, right: 20, bottom: 30, left: 50},
      width = 960 - margin.left - margin.right,
      height = 500 - margin.top - margin.bottom;
  
  var parseDate = d3.time.format("%d-%b-%y").parse;
  
  var x = d3.time.scale()
      .range([0, width]);
  
  var y = d3.scale.linear()
      .range([height, 0]);
  
  var xAxis = d3.svg.axis()
      .scale(x)
      .orient("bottom");
  
  var yAxis = d3.svg.axis()
      .scale(y)
      .orient("left");
  
  var line = d3.svg.line()
      .x(function(d) { return x(d.date); })
      .y(function(d) { return y(d.close); });
  
  var svg = d3.select("body").append("svg")
      .attr("width", width + margin.left + margin.right)
      .attr("height", height + margin.top + margin.bottom)
    .append("g")
      .attr("transform", "translate(" + margin.left + "," + margin.top + ")");
  
  var data = [
              {date:parseDate("1-May-12"),close:500},
              {date:parseDate("5-May-12"),close:600}
              ];
  
    x.domain(d3.extent(data, function(d) { return d.date; }));
    y.domain(d3.extent(data, function(d) { return d.close; }));
  
    svg.append("g")
        .attr("class", "x axis")
        .attr("transform", "translate(0," + height + ")")
        .call(xAxis);
  
    svg.append("g")
        .attr("class", "y axis")
        .call(yAxis)
      .append("text")
        .attr("transform", "rotate(-90)")
        .attr("y", 6)
        .attr("dy", ".71em")
        .style("text-anchor", "end")
        .text("Price ($)");
  
    svg.append("path")
        .datum(data)
        .attr("class", "line")
        .attr("d", line);
  
new_contents: var margin = {top: 20, right: 20, bottom: 30, left: 50},
      width = 900 - margin.left - margin.right,
      height = 500 - margin.top - margin.bottom;
  
  var parseDate = d3.time.format("%d-%b-%y").parse;
  var data = [
              [
               {date:parseDate("1-May-12"),value:500},
               {date:parseDate("3-May-12"),value:450},
               {date:parseDate("5-May-12"),value:600}
              ],
              [
                {date:parseDate("2-May-12"),value:400},
                {date:parseDate("3-May-12"),value:200}
              ],
              [
               {date:parseDate("1-May-12"),value:400},
               {date:parseDate("5-May-12"),value:200}
              ]
             ];
  
  
  var x = d3.time.scale()
      .range([0, width]);
  
  var y = d3.scale.linear()
      .range([height, 0]);
  
  // add scale of the axes
  x.domain(d3.extent(data[0], function(d) { return d.date; }));
  y.domain([0, d3.max(data.map(function(d) { return d3.max(d, function(dm) { return dm.value; }); } )) * 1.1]);
  
  var xAxis = d3.svg.axis()
      .scale(x)
      .orient("bottom");
  
  var yAxis = d3.svg.axis()
      .scale(y)
      .orient("left");
  
  var line = d3.svg.line()
      .x(function(d) { return x(d.date); })
      .y(function(d) { return y(d.value); });
  
  var svg = d3.select("body").append("svg")
      .attr("width", width + margin.left + margin.right)
      .attr("height", height + margin.top + margin.bottom)
    .append("g")
      .attr("transform", "translate(" + margin.left + "," + margin.top + ")");
  
  var lines = svg.selectAll("g").data(data);
  
  //for each array, create a 'g' line container
  var aLineContainer = lines
    .enter().append("g"); 
  
  aLineContainer.append("path")
  .attr("class", "line")
  .attr("d", line);
  
  svg.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis);
  
  svg.append("g")
      .attr("class", "y axis")
      .call(yAxis)
    .append("text")
      .attr("transform", "rotate(-90)")
      .attr("y", 6)
      .attr("dy", ".71em")
      .style("text-anchor", "end")
      .text("Price ($)");
subject: add several lines to standalone chart