Area Update Pattern by HarryStevens

index.html

<!DOCTYPE html>
<html>
<head>
  <style>
    body {
      margin: 0;
    }
    .area {
      fill: steelblue;
    }
  </style>
</head>
<body>
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <script src="https://unpkg.com/d3-marcon@2.0.2/build/d3-marcon.min.js"></script>
  <script src="https://unpkg.com/jeezy@1.11.0/lib/jeezy.min.js"></script>
  <script>

  var m = d3.marcon().width(window.innerWidth).height(window.innerHeight);

  m.render();

  var width = m.innerWidth(), height = m.innerHeight(), svg = m.svg();

  var parseTime = d3.timeParse("%d-%b-%y");

  var x = d3.scaleLinear()
      .rangeRound([0, width])
      .domain([1, 100]);

  var y = d3.scaleLinear()
      .rangeRound([height, 0])
      .domain([0, 100]);

  var area = d3.area()
      .x(function(d) { return x(d.day); })
      .y1(function(d) { return y(d.value); })
      .y0(y(0))
      .curve(d3.curveCatmullRom.alpha(0.5));

  drawChart(makeData());

  d3.interval(function(){
    drawChart(makeData());    
  }, 100);

  function drawChart(data){

    var area_path = svg.selectAll(".area")
        .data([data], function(d, i){ return i; });

    area_path
      .transition()
        .attr("d", area);

    area_path.enter().append("path")
        .attr("class", "area")
        .attr("d", area);

  }

  function makeData(){
    var arr = [];
    var start = jz.num.randBetween(45, 55);
    for (var i = 1; i <= 100; i++){
      arr.push({day: i, value: jz.num.randBetween(start, start + 20)})
    }
    return arr;
  }

  </script>

</body>
</html>