D3 Examples - simple line chart from dataset

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    .line {
      fill: none;
      stroke: black;
      stroke-width: 1.5px;
    }
    
    .tooltip {
        position: absolute;
        text-align: center;
        font: 15px sans-serif;
      pointer-events: none;
        }
  </style>
</head>
​
<body>
  <script>
​
    var svg = d3.select("body").append("svg")
      .attr("width", 960)
      .attr("height", 500);
​
    var width = 800;
    var height = 400;
​
    // See formats https://bl.ocks.org/zanarmstrong/raw/05c1e95bf7aa16c4768e/
    // Data sample {"name": "G", "value": 10, "date": "2016-07"}
    var parseDate = d3.timeParse("%Y-%m");
    var displayDate = d3.timeFormat("%b %y");
    var displayValue = d3.format(",.0f");
    
    // Ordinal scale
    var x = d3.scaleTime()
        .range([0, width]);
​
        // Linear scale
    var y = d3.scaleLinear()
        .range([height, height - 200]);
​
    var line = d3.line()
        .x(function(d) { return x(d.date); })
        .y(function(d) { return y(d.value); });
    
    var g = svg.append("g")
        .attr("transform", "translate(50, 0)")
    
    d3.json("dataset.json", function(error, data) {
        if (error) throw error;
​
      // Pre-processing
      data.forEach(function(d) {
                d.value = +d.value;
        d["date"] = parseDate(d["date"]);
      });
      
      x.domain(d3.extent(data, function(d) { return d.date; }));
            y.domain([0, d3.max(data, function(d) { return d.value; })]);
      
      svg.selectAll(".label").data(data).enter()
       .append("text")
        .attr("class", "label")
        .text(function(d, i) { return displayDate(d.date); })
        .attr("y", 420)
        .attr("x", function(d) { return x(d.date); })
        .style("font-size", 10)
        .style("font-family", "monospace");
​
      // Display last value
      g.selectAll(".value").data([data[data.length -1]]).enter()
       .append("text")
        .text(function(d, i) { return displayValue(d.value); })
        .attr("class", "value")
        .attr("y", function(d) { return y(d.value); })
        .attr("x", width + 10)
        .style("font-size", 20)
        .style("font-family", "monospace");
​
      g.selectAll(".line").data([data]).enter().append("path")
        .attr("class", "line")
        .attr("d", line);
      
      /* added code */
      g.selectAll("circle").data(data).enter().append("circle")
        .attr("cx", function(d) { return x(d.date) ;})
        .attr("cy", function(d) { return y(d.value) ;})
        .attr("r",  10 )
        .attr("fill", "#999999")
        .attr("fill-opacity","0.4")
        .on("mouseover", function(d,i) {
                d3.select(this).style("fill","red")
                               .style("fill-opacity","1.0");
                div
              .style("display", "inline")
              .text(d.value)
                  .style("left", x(d.date) + 50 + "px")
              .style("top",  y(d.value) - 20 + "px");
                pointille
              .attr("x1", x(d.date))
              .attr("y1", y(d.value))
              .attr("x2", x(d.date))
              .attr("y2", y(0))
              .attr("style", "stroke:black;stroke-width:1;stroke-linecap:square;stroke-dasharray:5, 4");
            })
        .on("mouseout", function() {
                d3.select(this).style("fill","#999999")
                           .style("fill-opacity","0.4");
                div.style("display","none");
                pointille
              .attr("style", "none");
            });
      
    });
    
      var div = d3.select("body").append("div")
        .style("display", "none")
             .attr("class", "tooltip");
    
    var pointille = g.append("line");
​
​
  </script>
</body>
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