title: Drawing multiple real time lines
body: <p>I want to draw multiple real time lines using JSON files. I am basically retrieving the JSON file from a website, getting the time data (duration in seconds), converting them into minutes and pushing them into the data array. This code checks the JSON file for every second.</p>
  
  <p>I want to add as many line as possible. For example, I want to add the average of the elements in data array  (average duration) and plot it on the same plane. I tried to add another "line" and "path" variable, however I wasn't able to plot it at the same time.</p>
  
  <p>The data array is an empty array with 44 elements in the beginning, and everytime the code checks the JSON file it replaces those zeroes with the retrieved duration data.</p>
  
  <p>Here is my code to draw only one line.</p>
  
  <pre><code>  function graph() {
  
    var n = 43,
        duration = 1000,
        now = new Date(Date.now() - duration),
        count = 0,
        data = d3.range(n).map(function() { return 0; });
  
    var margin = {top: 10, right: 20, bottom: 30, left: 60},
        width = 1200 - margin.left-margin.right,
        height = 460 - margin.top - margin.bottom;
  
    var x = d3.time.scale()
        .domain([now - (n - 2) * duration, now - duration])
        .range([0, width]);
  
    var y = d3.scale.linear()
        .range([height, 0]);
  
    var line = d3.svg.line()
        .interpolate("basis")
        .x(function(d, i) { return x(now - (n - 1 - i) * duration); })
        .y(function(d, i) { return y(d); });
  
    var line2 = d3.svg.line()
        .interpolate("basis")
        .x(function(d, i) { return x(now - (n - 1 - i) * duration); })
        .y(function(d, i) { return y(d); });
  
    var svg = d3.select("body").append("p").append("svg")
        .attr("width", width + margin.left + margin.right)
        .attr("height", height + margin.top + margin.bottom)
        .style("margin-left", -margin.left + "px")
      .append("g")
        .attr("transform", "translate(" + margin.left + "," + margin.top + ")");
  
    svg.append("defs").append("clipPath")
        .attr("id", "clip")
      .append("rect")
        .attr("width", width)
        .attr("height", height);
  
    var axis = svg.append("g")
        .attr("class", "x axis")
        .attr("transform", "translate( "+margin.left+"," + height + ")")
        .call(x.axis = d3.svg.axis().scale(x).orient("bottom"));
  
  
    var yaxis = svg.append("g")
        .attr("class", "y axis")
        .attr("transform", "translate(" + margin.left + ",0)")
        .call(y.axis = d3.svg.axis().scale(y).orient("left"));
  
  
    d3.select(".y.axis")
        .append("text")
        .text("Travel Time (min)")
        .attr("text-anchor", "middle")
        .attr("transform","rotate( -90, 200, 0)")
        .attr("y",-250);
  
    var path = svg.append("g")
        .attr("clip-path", "url(#clip)")
        .attr("transform", "translate(" + margin.left + ",0)")
        .append("path")
        .data([data])
        .attr("class", "line");
  
    tick();
  
    function tick() {
  
      d3.json("route.json",function(barzo){
        var tempdata = barzo.route;
        var len = tempdata.realTime;
        var lastdata = parseInt(len)/60; //this is the time variable I use.
  
      // update the domains
      now = new Date();
      x.domain([now - (n - 2) * duration, now - duration]);
      y.domain([0, d3.max(data)+5]);
  
     // push the time into the data
      data.push(count);
      count = lastdata;
  
      // redraw the line
      svg.select(".line")
          .attr("d", line)
          .attr("transform", null);
  
      // slide the x-axis left
      axis.transition()
          .duration(duration)
          .ease("linear")
          .call(x.axis);
  
      yaxis.transition()
          .duration(duration/10)
          .ease("linear")
          .call(y.axis);
  
      // slide the line left
      path.transition()
          .duration(duration)
          .ease("linear")
          .attr("transform", "translate(" + x(now - (n - 1) * duration) + ")")
          .each("end", tick);
  
  
      // pop the old data point off the front
      data.shift();
  
    });
    }
    };
  </code></pre>