draw.js

function draw(){
  ctx.fillStyle = "rgb(32, 32, 32)";
  ctx.fillRect(0, 0, canvas.width, canvas.height);

  var data = [20, 40, 50, 60, 30, 10, 20, 40, 70, 5, -1, -20, -5, -20, -7, -5, 5];
  ctx.fillStyle = "rgb(255, 255, 255)";

  var i = 0;
  var offset = 5;
  var width = (canvas.width - offset * (data.length + 1))/ data.length;
  while(i < data.length){
    var x = offset * (i + 1) + width * i;
      ctx.fillRect(x, canvas.height / 2 - data[i] * 4,

                   width, data[i] * 4);
      i = i + 1;
  }
}