var arr = [
  { id: 1001, price: 100 },
  { id: 1002, price: 150 },
  { id: 1003, price: 200 },
  { id: 1004, price: 70 },
  { id: 1005, price: 300 }
 ];
 var gap = 20;
 var canvas = document.querySelector("canvas");
 var ctx;
 init();
 function init() {
  canvas.width = 400;
  canvas.height = 300;
  ctx = canvas.getContext("2d");
  var max = arr.reduce((value, item) => {
  return value < item.price ? item.price : value;
  }, arr[0].price);
  //max高為300的4/5,其他的高為:300*(4/5)/(max) * h maxh:240 = othersh: ? ? = 240 
  var scaleHeight = 300 * 4 / 5 / max;
  //每個柱狀圖的寬為總寬-間隙寬除個數(shù)
  var width = (400 - (gap * (arr.length + 1))) / arr.length;
  createChart(width, scaleHeight);
 }

 function createChart(w, hs) {
  ctx.fillStyle = "rgba(0,0,0,0.7)";
  ctx.fillRect(0, 0, 400, 300);
  var x = 0;
  for (var i = 0; i < arr.length; i++) {
  x += gap;
  ctx.fillStyle = "orange";
  var h = hs * arr[i].price;
  ctx.fillRect(x, 300 - h, w, h);
  x += w;
  }
 }