//确定画布的大小
var width = 400;
var height = 400;
//在 body 里添加一个 SVG 画布 
var svg = d3.select("body")
  .append("svg")
  .attr("width", width)
  .attr("height", height);
//定义画布周围空白的地方
var padding = {left: 30, right: 30, top: 20, bottom: 20};
//定义一个数组
var dataset = [10, 20, 30, 40, 30, 20, 10, 5];
//x轴的比例尺
var xScale = d3.scale.ordinal()
  .domain(d3.range(dataset.length))
  .rangeRoundBands([0, width - padding.left - padding.right]);
//y轴的比例尺
var yScale = d3.scale.linear()
  .domain([0, d3.max(dataset)])
  .range([height - padding.top - padding.bottom, 0]);
//定义x轴
var xAxis = d3.svg.axis()
  .scale(xScale)
  .orient("bottom");
//定义y轴
var yAxis = d3.svg.axis()
  .scale(yScale)
  .orient("left");
//矩形之间的空白
var rectPadding = 4;
//添加矩形元素
var rects = svg.selectAll(".MyRect")
  .data(dataset)
  .enter()
  .append("rect")
  .attr("class", "MyRect")
  .attr("fill", "steelblue")
  .attr("transform", "translate(" + padding.left + "," + padding.top + ")")
  .attr("x", function (d, i) {
   return xScale(i) + rectPadding / 2;
  })
  .attr("y", function (d) {
   return yScale(d);
  })
  .attr("width", xScale.rangeBand() - rectPadding)
  .attr("height", function (d) {
   return height - padding.top - padding.bottom - yScale(d);
  });
//添加文字元素
var texts = svg.selectAll(".MyText")
  .data(dataset)
  .enter()
  .append("text")
  .attr("class", "MyText")
  .attr("fill", "white")
  .attr("text-anchor", "middle")
  .attr("transform", "translate(" + padding.left + "," + padding.top + ")")
  .attr("x", function (d, i) {
   return xScale(i) + rectPadding / 2;
  })
  .attr("y", function (d) {
   return yScale(d);
  })
  .attr("dx", function () {
   return (xScale.rangeBand() - rectPadding) / 2;
  })
  .attr("dy", function (d) {
   return 20;
  })
  .text(function (d) {
   return d;
  });
//添加x轴
svg.append("g")
  .attr("class", "axis")
  .attr("transform", "translate(" + padding.left + "," + (height - padding.bottom) + ")")
  .call(xAxis);
//添加y轴
svg.append("g")
  .attr("class", "axis")
  .attr("transform", "translate(" + padding.left + "," + padding.top + ")")
  .call(yAxis);