Toggling element visibility in v4

This is a multiple line graph demonstrating how to show / hide elements by clicking on other elements. This was written using d3.js v4 and is a follow on to the multiple line graph example here.

This graph is part of the code samples for the update to the book D3 Tips and Tricks to version 4 of d3.js.

<!DOCTYPE html>
<meta charset="utf-8">
<style> /* set the CSS */

.line {
  fill: none;
  stroke: steelblue;
  stroke-width: 2px;
}

.axisSteelBlue text{
  fill: steelblue;
}

.axisRed text{
  fill: red;
}

.legend {
  font: 16px sans-serif;
  font-weight: bold;         
  text-anchor: start;
}

</style>
<body>

<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

// set the dimensions and margins of the graph
var margin = {top: 20, right: 40, bottom: 60, left: 50},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

// parse the date / time
var parseTime = d3.timeParse("%d-%b-%y");

// set the ranges
var x = d3.scaleTime().range([0, width]);
var y0 = d3.scaleLinear().range([height, 0]);
var y1 = d3.scaleLinear().range([height, 0]);

// define the 1st line
var valueline = d3.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y0(d.close); });

// define the 2nd line
var valueline2 = d3.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y1(d.open); });

// append the svg obgect to the body of the page
// appends a 'group' element to 'svg'
// moves the 'group' element to the top left margin
var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform",
          "translate(" + margin.left + "," + margin.top + ")");

// get the data
d3.csv("data4.csv", function(error, data) {
  if (error) throw error;

// format the data
  data.forEach(function(d) {
      d.date = parseTime(d.date);
      d.close = +d.close;
      d.open = +d.open;
  });

// scale the range of the data
  x.domain(d3.extent(data, function(d) { return d.date; }));
  y0.domain([0, d3.max(data, function(d) {return Math.max(d.close);})]);
  y1.domain([0, d3.max(data, function(d) {return Math.max(d.open); })]);

// add the valueline path.
  svg.append("path")
      .data([data])
      .attr("class", "line")
      .attr("id", "blueLine")
      .attr("d", valueline);

// add the valueline2 path.
  svg.append("path")
      .data([data])
      .attr("class", "line")
      .attr("id", "redLine")
      .style("stroke", "red")
      .attr("d", valueline2);

// add the X Axis
  svg.append("g")
      .attr("transform", "translate(0," + height + ")")
      .call(d3.axisBottom(x));

// add the Y0 Axis
  svg.append("g")
      .attr("class", "axisSteelBlue")
      .call(d3.axisLeft(y0));

// add the Y1 Axis
  svg.append("g")
      .attr("class", "axisRed")
      .attr("transform", "translate( " + width + ", 0 )")
      .call(d3.axisRight(y1));

// add the blue line legend
  svg.append("text")
     .attr("x", 0)             
     .attr("y", height + margin.top + 15)    
     .attr("class", "legend")
     .style("fill", "steelblue")         
     .on("click", function(){
       // determine if current line is visible
       var active   = blueLine.active ? false : true,
       newOpacity = active ? 0 : 1;
       // hide or show the elements
       d3.select("#blueLine").style("opacity", newOpacity);
       // update whether or not the elements are active
       blueLine.active = active;
     })
     .text("Blue Line");

// add the red line legend
  svg.append("text")
     .attr("x", 0)             
     .attr("y", height + margin.top + 35)    
     .attr("class", "legend")
     .style("fill", "red")         
     .on("click", function(){
       // determine if current line is visible
       var active   = redLine.active ? false : true,
       newOpacity = active ? 0 : 1;
       // hide or show the elements
       d3.select("#redLine").style("opacity", newOpacity);
       // update whether or not the elements are active
       redLine.active = active;
     })
     .text("Red Line");

});

</script>
</body>

https://d3js.org/d3.v4.min.js